Your career earnings are an accumulation of all the credits that you win from a number of sources. First you get credits when you are promoted from one tier to another. You also can get credits and / or merchandise by qualifying for special tournaments, winning tournaments, closest to the hole tournies and if your state and / or country allows it by winning MPC (match play for credits).
You can also get credits by doing surveys, signing up for things such as netflix, buying from addidas online although these don't count toward career earnings.