This is an easy guide for Teemo that will help players with a little bit of effort to do exceptionaly well in games! It's focused on lots of attack speed and a balanced amount between attack damage and ability power and on-hit effects which makes Teemo the perfect destruction weapon! Also a small ammount of crit-chance will surprise your enemies!
This is my first guide so please be kind! xD
Also leave a comment and tell me your thoughts!

Pros:
Brute Force

+Incredible amounts of damage in a relatively small ammount of time!
+Quite good early game, excels at solo lanes where he is capable of farming and taking all the exp!
+He can annihilate tanks! (With this build)
+His mushrooms can give a special advantage to his team by placing in key places on the map and giving it vision!
+He one of the fastest champions in the league!
+His mushrooms are very useful because they slow enemy champions deal damage to them quite a few more!
+Quite good pusher!
+He can farm realy realy well with his mushrooms and his E ability!

Cons:
Arcane Knowledge

-He is fairly squishy.
-Is mainly focused during teamfights.
-Low base hp.

I choose flat crit-chance Greater Mark of Critical Chance and Greater Seal of Critical Chance because they can give the Teemo players these extra damage with the tons of attack speed this build gives and help them even more to kill a player when they wait for an ambush! For the first-blood for example with the attack speed from his passive and the crit-chance from the runes is a deadly combo!
Also i like to take flat magic resistance Greater Glyph of Magic Resist because since you are going in mid lane you are propably going to be against an ap champion so these runes are going to grant a lot of survivability!
I also take flat armor pen Greater Quintessence of Desolation because you have TONS of attack speed which means that you are mostly going to basic attack the other person and with the help of the armor pen you are going to be melting everyone!

For masteries i focus on the Offense tree, taking everything for ad and also taking the cd reduction and magic pen masteries while putting one single point in the Utility tree at the improved recall so you can return to base and be back in action in less time!
These masteries help melt through enemies hp like lava through ice! xD

* I try to level up Teemo's Toxic Shot (E) as fast as possible cause this is my primary damage ability! It takes along well with other on-hit effects and its with it's poison its deadly as hell! xD

*His (Q) ability is Blinding Dart. It makes the enemy's attack to miss for an amount of time and it is an ability power nuke which can deal damage and help your team out to make the enemy's ad carry useless for an amount of time!!! I try to level up this second!

*His (W) ability is Move Quick. It gives passively movement speed and has an active which if poped it doubles the passive movement speed! It is exceptionaly good for many things, like for ganking catching up to a nearby teamfight catching up to enemies or escape from a gank!

*His ultimate (R) is Noxious Trap. Teemo lays down a mushroom which is good for a lot of things! Firstly it can significantly slow enemy champions if they step on them and damage them over time as well! They give vision around them while they are placed down! (Something like wards but by dealing damage) They can prevent ganks, help you and your teammates escape, or help kill someone!

*His passive is Camouflage! It has a whole section about it which will be mentioned later!

For the items i like to start with a Doran's Blade which helps a lot in the early game with the extra damage and also gives a lot of sustain because it gives 80 health and 3% lifesteal which combined with your masteries makes it 6% lifesteal which is something great early game!

* I continue with Berserker's Greaves for the extra attack speed!

* Hextech Gunblade is an extremely useful item on Teemo because it gives both attack damage and ability power and also has lifesteal and spell vamp of which he benefits from both! It even has an active that can help him kill enemies faster!

* Nashor's Tooth is a must on Teemo because it gives attack speed (which is what this build focuses on) a relaively good amount of ability power, mana regen and has 25% cooldown reduction!

* Malady is also a very good item on Teemo because it gives a big amount of attack speed a little bit of ability power and deals passively 20 bonus magic damage on each hit and shreds the enemy's magic resistance which means that you will be doing more dmg on each hit and with your abilities!

* Next item is the Madred's Bloodrazor. It's an awesome item because it gives attack speed, armor, some attack damage, and has an amazing on-hit effect! It deals 3% of the enemy's health points on magic damage! And with all that attack speed it's lethal!

* Now for the last item i buy a Wit's End! They are both great items! The Wit's End gives a significant amount of attack speed and some magic resistance which helps a lot if you are being focused by ap casters! It also haves an on-hit effect which deals 42 magic damage on each hit and it's passive gives 5 magic resistance on each hit, which stacks up to 4 times (Conclusion? Deadly item!)

I like taking Ignite and [Exhaust on Teemo!! I take Ignite in order to finish off injured enemies, help me kill them, and generally make my life easier! xD I use Exhaust for many reasons! Firstly, it can help me win those 1v1 in mid lane, or help me get away from someone that is chasing me, escape ganks or even win a 2v1 battle by using Exhaust on the one of the two and blinding the possible ad that is fighting you!!!
Other useful options are Ghost or Flash! The Ghost in order to get away faster or reach a nearby teamfight faster! It also synergises well with your (W) ability Move Quick which makes Teemo faster than the speed of light!!!! xD You could use Flash as well for almost the same reasons as Ghost!!

Teemo with this build is made to be an assasin! He can annihilate almost anyone that tries to 1v1 with him! Especialy if the one who attacks you is an ad that counts on basic attacks, or generaly an ad! He can avoid almost all their damage that comes by their basic attacks with the help of his (Q) ability Blinding Dart! And by the time the blind effect has worn off your opponent will have either died or be alive with realy low hp while you will be with almost full hp!
Now on mid lane try to harass as much as possible with basic attacks in order to bring the enemy on low hp! If the enemy tries to attack you don't panic! Use Exhaust immediately to lower their damage that will deal to you and also prevent them from escaping! Use Ignite as well to bring them down easier!

That's the thing that i can say i like the most on Teemo, his passive Camouflage! If Teemo doesn't take any actions or move for 2 seconds he turns invinsible for as long as he wants! Now you can use this for many many many things! First of all you can turn yourself into a ward! ahahahahahah!! Secondly can prepare ganks by sitting in a place and waitng for an ambush with or without the help of your teammates! You can even escape when you are being chased! For example let's say that a wild Ryze is chasing you and you are with really low hp! What do you do? You firstly find the nearest bush! Then you immediately stop moving as you reach the bush in order for your passive Camouflage to take place! And lastly you wait there until the enemy leaves! Or if they know that you are in the bush what you can do is wait a bit and then search for the route you can take which is the less crowded! For example lets say you are in the bush next to the dragon, and the enemies know that you are there! You wait a bit and then you pop your W Move Quick for the extra movement speed and escape like nothing happened because almost noone can follow you with this extreme movement speed you have! Oh somehing i forgot to mention! His passive Camouflage when you get out of stealth you get 40% increased attack speed which helps in ambushes even more!!!!

After you spawn in the game you want to immediately buy your Doran's Blade as fast as you can and run to mid lane! If you don't have a jungler this should work! You run as fast as you can in mid lane right in front of the enemy's turret vision! You stay there and let your passive take effect! Then if you are lucky enough an enemy will come! You wait until he is right two or more steps behind you towards your turret! When this happens you immediately pop out attacking him with your basic attacks along with your Exhaust and Ignite in order to bring him down easily! But watch out do not do this if the enemy minions are next to you or you will get minion agro and die along with the enemy's help!

Farming with Teemo is pretty easy because he has his ulti Noxious Trap which can be used to effectively farm when there are a bunch of minions around! He can also farm well with the on-hit effect and attack speed from this build and the on-hit effect from his E ability passive and it's poison as well!
If you farm up well you will have a great advantage in the game that's why you have to focus on farming early on rather than try to kill! (not that if the opportunity comes to kill someone just farm and let the kill go away)

That build was made according to my oppinion and how it turned out to me while playing it! This was my first guide and sorry if i did something wrong or i missclicked and wrote a wrong letter or things like that because this is a common problem! I had quite much fun creating it and i hope that you will have as much fun as i did while reading it! Also leave me your thoughts about this build and how it worked on you if you tried it, or even why you are not going to try it!
My credits to JhoiJhoi who helped me create this guide!
Peace out!