I see a number of people have just started playing so I thought it might be useful share a few things I wish I'd known when I started.
- First half of the game, or until you reach Lv10 or so, there's not much to do except defend your tower and stay behind your minions to get your exp and gold.
- Time your attacks on minions to get the killing blow, you get more gold that way.
- Don't over-commit if you try to go for a kill, it will very often end with you being killed instead.
- Until you learn your champion, buy the recommended items, they are designed to help people that are starting out. It's usually a combination of health/mana regeneration to start with and then the more powerful items for later during the game.
- Always call and inform the rest of your team if enemies in your lane go missing. They are very likely trying to setup a gank and your team needs to know. All you need to call is: Top/Mid/Bot MIA - depending on which lane you're in.
- Do not go off alone if you cannot see where the enemies are. This is especially important later in the game where your chances of survival are just about nil if you come across more than one enemy.
This is by no means comprehensive, so if there are any questions, I'm happy to answer...I have been playing it quite a bit now