Hi Tamara,
There is a lot of information on this forum about flocculation - just search for it in the search box on the top right hand corner.
In brief, you float a small plastic container in your mix as soon as you add the rennet and write down the exact time. The milk will quite suddenly 'set', at which point (anytime from a couple of minutes until 20 minutes or more later) the plastic container will not move when you gently push it with your finger - you need to keep a close watch and test it frequently after a few minutes. This is the flocculation point. You then carefully note the time elapsed since you added the rennet, multiple this by a magic number which varies according to the type of cheese. Wait for this amount of time to elapse from the time of adding the rennet (note: not since the flocc. time, a commonly made mistake!) then try your curd for a clean break. Sometimes you might judge it sensible to wait a little longer if it seems too soft, but it's usually spot on. This might sound complicated but once you get your head around it it's easy. The science behind it is more complicated than I could try to explain, others have done that far better than I could, but it's all to do with the interaction of milk, rennet, culture and type of cheese.
Margaret