Swain is an actual beast, you'll most likely be underestimated and your demonic flare will mess them up.

Passive - Pull whenever you can, and remember, you're able to pull ANY cc so if your teammates cc don't forget to pull! Stacks can be the difference between a kill or your death. Always aim to build five.

Q - Careful with your mana use, but don't be afraid to use it a lot. It restores its own mana so try to aim to both hit the enemy and kill many minions.

W - Landing it in narrow routes are guaranteed stacks. Stacks are important.

E - It's all practice but once you learn to land this you should be set. Practice and know your range.

R - Don't be afraid to use it. Most of the time you win 1v1s or 1v2s but it starts with the ult so don't be scared! Fight!

Best bans include:

Nasus - He's tanky and scales so well late game he can carry a game if you mess up or reach late game. A basic top lane ban but a needed one.

Renekton - Too strong, you just can't win if he gets ahead so just be safe.

Don't tilt.
You're going to win as long as you play smart and don't fall into the dark spot in your head. Just have fun and play the game, it's just a game. Swain is just so amazingly strong he can carry games as long as you AND your team doesn't tilt. So do them the favor and encourage them, tell them it's gonna be alright :D