Is there a certain size tube for my tyres?
Each tube reference size fits a certain number of tire sizes.
For example, I ride 700x23c tires and my tubes are 700-18-23c.

So just find out the size of your tire (should be written on the sidewall) and go out for a tube that will hav this tire size in its fitting range.

Be careful about the valve specification too. get a tube with the same valve type as your previous tube/as your wheel supports : http://sheldonbrown.com/gloss_u-v.html#valve