1. Win your lane - General rule of thumb is win your lane and win the game.
2. Help other lanes win their lane - If you're playing top and you've just taken the turret and the enemy top laner is mia or dead, then go to a different lane (Mid, for example) and help them kill the enemy mid laner and take the turret.
3. Play OP champions - You're going to have a much better chance of winning games if you're playing what people would consider an OP champion.
4. Wards - Buy wards regardless of what role you are playing. Wards win games. Every time you go back to base buy 1 or 2 wards, they will most likely save you/ a team mate or help you secure a kill on the enemy jungler who is trying to steal your red buff.
5. Try and play with friends - Playing with a friend or multiple friends helps remove another random player from your group which means you're going to have more communication and more trustworthy and consistent players.
6. Creep score - Always. Always. Always last hit a creep, the better you get at this the more you will be able to get yourself a full build quicker. A good goal for creep score is about 70-80 by the 10 minute mark.
Those are my tips on how to win, they work for me so hopefully they'll work for you and anyone else too.