I tried to find this as I think it's a common issue but could not in the search.
I own a .com domain which is my main site but I also own other tlds for the same domain name such as .net, .mobi, .org, etc.
Is there something I should be doing to configure them somehow to be apart of my main .com site or should I just sit on them?
.com
, you can serve different content through them or you can just park them and not make them resolve. This all depends on many other points you did not describe.