Donna, there are several categories of rewards systems on WikiTree.
Many people probably first look to their badges. They are displayed on your profile, although you have the option to not display all but the green "Wiki Genealogist" badge. You can also rearrange your badges, except for that initial one.
Any time you make at least 100 contributions in a month, you will get a badge for that. If you make 1000 contributions in a month, you will get a different badge that some people value more highly. Over time, you might begin to consider a person's total contributions as more important than the particular badge they earned in any given month.
Contributions, however, are not an entirely reliable guide to, well, a person's contributions! They may come from a gedcom upload, or a lot of tiny changes by someone who saves every few minutes. Some people spend hours on a profile and then post their research all at once, and only get credit for one contribution. I sometimes get credit for a contribution just for fixing a single typo. I don't consider that cheating. If I see an obvious typo, I'll correct it, even if I don't have anything else to contribute to a profile.
You will also get badges for the projects you join, and for WikiTree responsibilities you volunteer for, such as being a Mentor or Ranger. I'd advise taking a little time to get used to WikiTree before doing those things.
You will get a thank-you any time someone decides to thank you for a contribution you've made. In my opinion, this is one of the least important features, because some people thank their friends 10 or 20 times a day and distort the entire system.
G2G is pretty much a separate part of WikiTree. The second link that Brian Kerr provided explains how points are earned. As with the other features, you might find it exciting to earn points at first, and maybe over time feel more secure in your position within WikiTree, and not pay so much attention to the rewards.