To file a form online you have to first create an account. Besides filing, this account allows you to check your online status, among other things. The USCIS website has a dedicated page for online forms, where you can learn about creating an account and tips for filing online.
The forms available for online filing include the AR-11, G-639, I-90, I-130, I-539, I-765, N-336, N-400, N-565, N-600, and N-600K. If you are immigrating to the U.S., there’s also a USCIS immigrant Fee, unless you are exempted.