Hmm, well first that's a good way to improve however. I'll give you a list of suggestions.
1. Know how to build your champions, look up guides on
mobafire.com or
solomid.net
The guides will give you in-depth tactics and skill building sequences for your champion.
2. Know your counters and know how play against them. Basically this is comes from experience but there are tips that can definitely help.
Go to
championselect.net for tips!
3. As you mentioned, watch videos on guides for certain champions possibly, or watch pro-players stream on twitch.tv
Players from different LCS (championship series) teams stream all the time. So just take a look whenever you'd like.
3.Practice, practice, practice! As they say practice makes perfect. It definitely will help you get better.
By gaining your own experience through playing League you will be able to know when to trade against players, farm properly, team fight better, know your own limits, and basically know the game.