RE: a bigginer's question...
If it's rolling onto its back and not always to the same side it's most likely a snap that is being caused by not getting enough airspeed before taking off and/or pulling back too much to take off. Gently does it!.
Take a look at the balance (both fore and aft and lateral), make sure the balance is where it says in the plans and that it is neutrally balanced 'sideways'.
If the balance is OK, wait a little more when taking off, to get more airspeed and/or be a little more gentle when pulling back on the stick.
Other than that, it could be a warped wing or an alignment problem with the tail. Look at the model from the front and see if there are any obvious mis-alignments.
Hope that helped!
-David C.