Since LoL is an amazingly popular game, there are also a ton of guides out there made by very good players. Be sure to check them out to learn good strategies such as which skills to upgrade first, which items to buy, runes and masteries, etc. Note that champions are tweaked constantly and so avoid using older guides that are now obsolete. Watching videos of pro players is also recommended.
Everyone has different playstyles and there are a ton of champions to play, so avoid stressing out of your first or second choice result in you getting your butt kicked. Either keep learning with that champion or find a different champion that better suits your thinking and playstyle. Since you get 10 heroes for free every week, you can also save up your IPs and then unlock a champion that you particularly like when you find one.