Swain is a champion not so easy to master and it might be hard to win your lane (or game) with him. Whatever, he stays a really fun champ and someone with a great potent!!! This guide will give you some tricks and advises to help you get better with that champ.

Pros:
-Great healing with ult
-Zone CC
-DoTs that can get down most of the ennemies
-VERY TANKY CHAMP
Cons:
-Low range spells and auto attacks
-Mana dependant
-Not many zone damage

My choice of runes isn't the one you will take, but it depends of what is your playstyle and what you prefer. I personnaly take:
Ability power glyphs: boosts ap, maybe the best choice for glyphs
Magic resist seals: Swain is a tanky champ, and getting some mr for him can make the change
Magic penetration marks: Great penetration for runes, adds with void staff and liandry's tourment
Ability power quintessences: Devastating.

A basical 21/0/9, with 9 in utility to help to solve mana problems. You can also get 21/9/0 to be more tanky.

Start with Boots and 3 pots, Swain needs some speed to get enough range to get ennemies.
Upgrade into sorcerer's boots after buying a large road (or immediately if you get not enough gold)
Then take immediately a rabadon's deathcap, it is much more effective than getting seculary like many swains do, for a similary price you get much more ap so much more healing with your ult. You can take seculary, but it doesn't work much.
After this rilay's scepter to get more life and slow with spells, because you need this not your target to go away, and it will combo with liandry's you should take just after, for magic penetration, that OP PASSIVEwith swain and MORE LIFE.
Then take abyssal scepter, just an item every swain should take, and to finish a void staff.

This is the core item I take for Swain, it's up to you to switch item orders or switch some item for another, because you shouldn't take the same stuff for each game situation.

PS: This stuff gets no mana boost, and Swain gets mana problems. If you can't get a blue by killing champs or killing the golem, you might get a mana item like Athene's.

IGNITE and FLASH. Why not switching Ignite for exhaust, it can work too (but less since season 3 patch).

Your W should be taken at level 1 because it can help to avoid ennemy invade or early ganks and help your jungler to make a level 2 gank. Moreover, I got many first bloods with cage ignite and auto-attacks.
Then upgrade Your E at level 5, then your Q and finish with your W, taking a point in R as soon as possible.

Be offensive, without overpushing. Farm and harass your ennemy with auto-attacks waiting for an opportunity to cage him with W and get the kill with the simple combo W->E->Q(+ignite if needed).
If you get ganked often, ward the bushes and play safer. As ganked, try to cage the jungler and go to your tower.

As your ult is ready, the game should become much more easier for you. Get your blue and you can farm with your ult. If you didn't get kills before, you will get less problems to do it. You can start ganking top and bot now because you have enought damage to burst both lanes champs.
If the first teamfight goes on, don't tank it (you should never tank, but now it's just impossible). You don't have enought hp and resists to tank and dmg. Just stay behind your tank(s) and try to get the ennemy carries with the W->E->Q combo.

If you got fed, the game is yours. The teamfights really start now and you get enought stuffed to damage the ennemy team and resist all fight long, why not even tanking with another champ if they don't get too much burst. Don't act too much crazily, play with team and the win is above.

Thanks for looking at this guide for Swain!!! Now you have most of the tools to pown your opponents! Don't hesitate to comment this guide (OR GIVE ME A GOOD OPINION :D). See you in game!