Look at the "get credits" menu right above the big "FORUMS" line on this screen.
This being a commercial site, there ain't many ways to get credits for free - buy them with money or earn them by spending your time.
The only incentive you can get from WGT is to tier up - e.g. 50 cr for becoming an Amateur.
Some free tournaments have credit prizes - hard to win them w/o spending credits on equipment before...