Simple...practice. Don't play a Ranked game until after you play at least 3 normals within the last 12 hours, regardless of how high or low your elo is. This will reduce the chance of losing Elo from being rusty and give you a chance to play champs you aren't really the best at without your team flaming you.
Learn how to play/counter every champ to avoid being matched up against someone you haven't seen before. This is will also broaden your skill level to champs you probably never thought you'd do well with before.
learn how to play every lane. You don't have to the best at everything, but learn how to play them so you can't be forced into a position you haven't done before. If you say you have no good solo top champs or carries...go buy some. I would highly recommend you learn how to jungle properly, as this not only will help you with ganking lanes, but also teach you proper map awareness that is vital for any lane or role
WARD!!!! sight wards cost about 2 1/2 minions and pink wards cost about 4, so there's very little excuse why you shouldn't be able to pick up at least one. Buying Vision wards also gives you the bonus effect of being able to destroy enemy wards, leaving them in the dark. 1 vision ward > 3 extra health / mana pots.