As it has been said already learn 2-3 mid/top champions and play them a lot. Like 500 games lot.
Learn to play at least two of the following supports well - taric/leona/nunu/sona/lulu (1 poke and 1 tanky support would be best). so that you feel comforable when there is no one elso to play support.
When you lose 2 games in a row give yourself a break for the day - play dominion or aram or just dont play at all. If you continue to play you may end with a long loss streak (as you'll prolly end up raging which will have a negative impact on your play).