Unfortunately, the battlegrounds from 25-34 (braemer and wilton) are almost always empty. However, Thidranki (20-24) and then Molvik and Leirvik (35-39 and 40-44) are all usually well filled with plenty of action. You can get up to RR4 here, so by the time you go out to the big RvR, you have some toys to play with and know your class a bit.
I would suggest just level until you hit 35, then try Molvik.