Killing monsters and selling goods to other players are some very good ways to gain credits. Monsters give 1 Credit for being killed as long as you are not by a spawner or in a safe zone. To sell goods, either wait until someone is buying something or advertise what you have for sale in /ch t
The best way to make money as a starting player, however, is to mine gold. Gold is a constant of exchange and you can always get 12 Credits for gold at the DHX bank located in Sanctum, or even more from other cities who are looking to try to make a few C.