I decided to make a guide for this, as i've seen plenty of times when people are unsure which spells to take in which situations. i've seen many times where people do things like only take flash and ignite and nobody takes an exhaust or a heal or anything else.
i know full well that in ranked a lot of people will know their champions they use all around, but this guide is mainly pointed towards those who aren't experts.

IT IS ESSENTIAL THAT A TEAM TAKES A GOOD COMPOSITION OF SUMMONER SPELLS IN A MATCH. I CANNOT TELL YOU HOW MANY TIME THE SHEER FACT THAT SOMEONE TAKES AN IGNITE, HEAL, EXHAUST, GHOST, ETC.. WILL BE A BIG GAME CHANGER.

with that in mind, it is essential to learn every use for each summoner spell, and when to use it. even if you've been playing for a long time, there may still be things you haven't thought of, so do not dub this as a "guide for noobs"

I will admit, this is my first guide, i wanted to make a guide with some of my overall knowledge for my first guide rather than go in depth about one specific champion.
i hope that this guide may help anyone to better know what to take and when.

Please do not downvote, at least read this first. thanks <3
- I put alot of effort into this, do not downvote it if you think this is unnecessary, it is an overall guide meant to help in general, not with really specific champions or playstyles

Comment to vote, thanks :)

Smite

Smite

This summoner spell is pretty straight-forward for its use. Its used on junglers, and is a necessity for jungling, no exceptions. Good junglers will take the summoner's resolve mastery with this so that smite grants 10 gold on use, to help with the gold disadvantage sometimes found in jungling.
In RARE situations, another player may take a smite to help speed up your jungler's jungle speed, but i would really NOT recommend this.

Clairvoyance

Clairvoyance

This summoner spell is often used on support champions in bot lane. most other champions wont use this well. Its a great spell for gaining sight as a temporary "ward" to avoid ganks, or to give a sight advantage in teamfights if the enemy champions are going in and out of the brush.
In some especially conditional situations - you can use this on champions with a global skillshot like Ezreals Trueshot Barrage to know where to fire it and (if your lucky) get a kill.
This can also be used in the enemy's jungle to know where the enemy jungler is to counterjungle - ask your support to take clairvoyance if you plan on counter jungling
Good places to use this is in front of the dragon enough that you will gain sight of the entrance from the jungle, or the same for baron. If you are unsure where to use clairvoyance than please consult Panglot's warding helper as to know general areas you want to keep sight of :) .
A tip of advice; Clairvoyance is a great tool for checking if Red/Blue buff is taken, so tell your jungler to ask you to check them if he needs it.

Exhaust

Exhaust

This summoner spell is great on almost all champions actually! having a champion with this in a match can make a huge difference with whether you/your ally gets the kill on a champion or not, or whether you escape a pursuing champion or survive his damage if you cant escape, or whether you can catch an escaping champion.
it also reduces defenses with the summoner's wrath mastery, which makes a big difference.
It can help catch a speedy champion like Singed, or stop a Tryndamere who just Undying Raged from destroying your team and possibly buying you the time to kill him when his Undying Rage ends :)

Ignite

Ignite

This summoner spell is great for getting the last ticks of damage to get a kill (is especially great for getting first blood), or when facing a champion with heavy sustain.
it reduces healing on the champion by 50%, so keep in mind you CAN use it to stop a support champion from healing up the champion so you can get the kill on him/her (only do this ever if you KNOW you/your ally WILL get the kill).
This is especially useful against champions like Dr. Mundo, especially when he uses his Maximum Dosage

Revive

Revive

Like most people stereotypically say - this spell is not good. ALTHOUGH there are exceptions. its generally a horrible spell because its for use when you die - and you do not want to die at all, however, as a tank, this can save your team lategame.
let me explain;
if you are the tank late-game and you die during a teamfight, then your team is screwed for that teamfight,
using revive will let you run back into the fight and save them (if you get there in time).
This spell is so conditional and risky though, and has such a long cooldown, your better off taking heal if you want to stay alive, not this.

Ghost

Ghost

Ghost is a GREAT summoner spell and is often unused in favor of flash. It can give you the ability to escape, or chase down a running enemy champion. This spell is great for melee champions who want to catch a ranged champion running away, or for champions with no escape mechanisms. Its especially good on certain champions that synergize with movement speed (like Hecarim or Singed).

Cleanse

Cleanse

This summoner spell is amazing and often underused. it removes any crowd control effects on you, and reduces the duration of incoming crowd control by a whopping 65%.
This spell is great if your a champion who gets destroyed if they get cc'd, or for anyone who gets focused alot. champions with really powerful ultimates like Tryndamere or Master Yi may be the target of crowd control effects often, and may want to take this.

Surge

Surge

Again, another underestimated summoner spell. This spell is situational as to what champions can use it well. i personally would not normally recommend it, but it can be good on AD/AP hybrid champions or AP/AS hybrid champions (like possibly a Teemo ).

Teleport

Teleport

Another GREAT summoner spell, used a lot on champions who solo top lane.
This spell is great for when you need to recall or you die, and you need to get back to your lane fast. this can save turrets from dying VERY often.
Ashe has a funny combo with this summoner spell, in which she can use her Enchanted Crystal Arrow at the base, then Teleport close to the champion thats about to get hit and pick up the kill.
Draven also has a funny combo with Teleport, although impractical, after he Whirling Deaths and his Whirling Death is about to come back to him, at the last minute he can teleport to another location and it will follow him back again, in which he can then recall and it will follow him again. do not try this in any serious match though, its very impractical.

Flash

Flash

One of THE best summoner spells for sure.
has many uses, chasing, escaping, if done right - you can flash over walls to get a huge gap in escaping, can be used to gank with certain champions like Maokai or escape certain spells like Jarvan IVs Cataclysm

Clarity

Clarity

This spell is highly underused, it can be GREAT for supporting you and the other person in your lane, this is ESPECIALLY great with champions that have huge mana problems early game.
using this on certain champions can give them the ability to harass with spells longer to push the enemy back and deny them creep kills with them trying to avoid your spells :D - works best with champions that have great ranged harass that does tons of damage.
i personally like this on Maokai so that you can spam Sapling Toss and often deny minion kills from your enemy (as people usually run from them to avoid the damage).
NEVER take this on a manaless champion - i hope this is not neccesary to say, it should be common sense ;p

Heal

Heal

Heal is a great summoner spell, for early sustain, or when facing a champion 1 v 1 who thinks he/she can win, using it at the last minute so you can get the kill and deny him/her the kill on you :)
usually used on an AD carry, but makes for a great support champion summoner spell too.

Promote

Promote

This is a very situational summoner spell, but is usually great for roamers who want to get gold still while they leave their lane, or for players who play a very aggressive playstyle and want to push hard (although, be very careful if that is your playstyle).
This summoner spell isn't used much, as "roamers" aren't popular in the current meta-game.
If you do use this, it would be used on someone like Evelynn i've found :p

Barrier and Garrison

Barrier is only available in custom matches so i wont go much more in depth than the fact that it is a great way to prevent yourself from getting killed
Garrison is only available in dominion, and is useful for defending turrets/taking turrets, as it kind of says in the effects of the spell itself ;P

Some summoner spells can be used very well together, it generally depends entirely on the champion being used.

Some GREAT examples of summoner spell combos are;

Heal/ Exhaust: Focused on sustaining yourself and your ally, and helping you/your ally get kills/escape from getting killed

Ignite/ Exhaust: I like to call this "First blood mode" ;) .
only take this if your a champion that really accelerates at dealing TONS of damage, as you can utilize this to get kills easier

Heal/ Clarity: most people wouldn't dare take this, and some will call "noob" at the sight of it, but it sustains your lane incredibly well - keeping your health and your mana up (as well as a half of that for your ally).
I use this bot lane if the other person bot with me already has Exhaust :)

Flash/ Ignite: a GREAT combo, very utility based and offensive. also great for getting first blood :). This summoner spell combination is VERY common i find.

Cleanse/ Ghost: an AMAZING combination; used when your champion needs to be able to escape/chase, makes it very hard for the enemy champion to stop you from chasing them/stop you from escaping

Teleport/ Ghost: I see this combination alot on solo top champions, this is great for solo lanes who want to be able to keep their turret protected and escape/chase when needed.

Exhaust/ Ghost: AMAZING for catching/escaping champions if you dont want to take cleanse with ghost to do so.

Thank you for reading my guide on summoner spells :)

Please up vote - or at least tell me why if you down vote, and if there is any suggestions/changes i can make, please say so in the comments.
I will edit this in the future if need be.

Suggest any good summoner spell combinations that i may have missed as well.

If there is anything i can do to make this guide better, please tell me :)

Thanks!