Mordred isn't RvR, it's PvP. With RvR you fight against people from other realms. Playing a PvP server means you fight against everyone. Isn't the best place to start, after level 10 anyone can just run up and kill you just because they feel like it.
If you want a server with a good balance there are many options, the best way is to look on the Camelot Herald and check populations on any given time. I play on Igraine, and all 3 realms go back and fourth as far as who is the better realm. For a few months it's Hibernia, then Albion, then it's Midgard. Just depends on your preference. :)