Totally agree with the above posters. Many new players get confused by credits and XP.
The credits can be earned http://www.wgt.com/user/offers.aspx
But prepare for disappointment and the knowledge you will have to write to the offer providers giving proof of what they should have given you.
Paying is the easiest option, if you can afford it.
Earning credits from tournaments......to be blunt is not really going to happen, and certainly not in the volumes you need to upgrade.
Be patient and wait as long as you can before upgrading - the earlier you join the gravy train, the more upgrades and useless purchases you will make.
Of course, if you can afford to upgrade constantly, another matter, but if not, just play away, enjoy the game and learn the different shots and courses - set yourself realistic goals with the equipment you have etc..
Have fun and enjoy the journey.