Either know at least 3 lanes or play one type of champ really well (tank, dps, burst, assassin, etc.). I find doing well in teamfights is usually better than doing well in lane, so if you know how to play, let's say, tanks, you'll know what to do with a mid Malphite once teamfights come around. --- Generation 30: The first time you see this, copy it into your own signature (on any forum) and add one to the generation number. Social experiment.
If you can't be decent with all 5 lanes, don't play ranked. Play normals and practice that position until you can play it right. There is no reason to not know how to play all positions, this game isn't hard.
You need to at least know all the roles, even if you aren't great at some of them. At least enough to the point where if you do happen to get stuck with it your team doesn't get the impression you're intentionally playing poorly.
You need to understand all the roles on a theoretical basis just to be able to be able to comprehend the strategy of it as a team based game, and you should be ready enough for the rare occasion where you happen to get stuck with your least favourite role.
Its not really that hard to do any of the roles at an acceptable level. Just play safe, don't die when it could have been avoided and coast along until laning is over. If you suck at teamfights, play some ARAM till you are familiar with all the champs you own mechanically.