YouTube does genuinely help. Once you get a handle on the co-op bots, you can learn a new champ pretty effectively, too. Mobafire.com is good for the general idea of a champ, like what kind of stats they like, but you can go towards the items you prefer when it gets down to specifics.
Based on the champs you chose, you seem to enjoy mid-laners or versatile champs a lot. As the free champ rotation changes, keep an eye out for mid-friendly champions that run off of AP, like LeBlanc or Veigar. The free rotation changes every Tuesday, so check out Mobafire and look at the kinds of champs you're dealing with. At your level, you have a lot of room to explore, so feel free to experiment in co-op! That's what it's there for. And don't shy away from mistakes. Anyone who rages out at you for them in co-op or unranked PvP isn't worth dealing with, so just ignore them, but do take friendly advice that comes your way!