ditto on what VegasTriker recommended. Read whole thing and understand what BSD is.

Once you've got that you will always be able to pick a correct size tire for your rims. The ISO size is stamped on every tire I've ever looked at. You might have to look hard as sometimes it is the smallest lettering. Rims don't always have their BSD or inside rim width on them, but if the tire you took off fit correctly, then you can get the BSD from it. You'll have to measure the internal rim width with calipers.