Not sure if it is the only way but if you compete in tournaments you get credits (not sure if its only when you are top 100 or whatever)
Play in the tournament.. a icon of a trophy and place one third of all total players and you will recieve a credit
You can win them in tournaments by placing in the top third or you can buy them...[to buy them click on the number of credits you have by your name and level and display picture near it says play and all that (top right hand corner) then scroll down to the bottom of the page it takes you to and click on one of the methods of payment and it will show you form there....]
U must play in the tournament and reach the quota (3rds of the rest of the player for the tournament)
or u can also buy them from the shop ...
You buy them. On the My Collection tab there is a Shop option click it at the bottom of the new page you can find the all the possible option for your country to buy credits. Or just click this link: http://www.urban-rivals.com/shop/
Or you can win them by playing during daily tournaments.