Please research the champion you are playing.

Especially if it's a new one, since you probably don't know how they work. Please just know the champion and what I mean with this is:

1. Knowing what their abilities do before getting into a game.
2. Knowing what to max first, what skill to take at level 1, 2, 3 and 4.
3. Knowing what runes to take and why.
4. Knowing what items to start with and how to itemize in general.

All these points can be found either on the champion's wiki page, or in sites like Probuilds, where it shows what pro players do with the champion for points 2, 3 and 4.

Recently played with a Janna that:

1. Opened with Q, which is useless. Opening W is best for extra MS and laning presence / damage at level 1.
2. She maxed Q, then E, then W. This baffled me beyond belief. Maxing Q is the last skill you max. She was essentially useless. Her W MS bonus - damage and slow were almost non-existent, her E shield - AD bonus was almost non - existent and her maxing-Q damage was unreliable. Essentially, a useless champion, barring her ult.
3. She bought Twin Shadows first, which is a useless item, at least for the early game. Ardent Censer and Redemption are, by far, the best starting choices. Twin Shadows isn't even good on Janna and it's a very situational buy.

She also had other issues regarding general champion mechanics and map awareness related, but those are not the point here.

Again, all these points above, she could have easily searched what a pro player builds and maxes and do the same, since she apparently had no idea. Oh, and this was in a ranked game, just so you know.