How to measure the size of a suitable tire for my bike? If I have the diameter of the rim of my bike and also have the width o this rim, how can I determine which tire is good for me?
Measure the diameter of the bead seat (i.e. the place where the tire bead will sit).  It will most likely be one of these:


559mm = MTB 26 inch tire.
622mm = 700C (a.k.a 29er)
630mm = 27 inch that is old enough to remember the Bee Gees.


If it's not one of those, look it up here:  http://sheldonbrown.com/tire-sizing.html#iso

The new tire will have a marking on it like 28-622.  That second number is the diameter.