Welcome! most of your answers can be found browseing the site, perhaps starting with http://marp.retrogames.com/rulespg.html(posted 9338 days ago)Screenshots are nice but not acceptable alone. You must have a .inp file that playsback. it's a good idea to make sure it playsback with the mame version you recorded it with before you upload. if there were special options you've used like: you've recorded without sound or with a special frameskip or if there was a nvram/gamename.nv file in your mame dir, put that in the upload description.
it's best to download some of the recordings you plan on uploading to, to see what settings previous users have used. and then go along with the flow. these settings may change, but it's best to keep everyone at an even playing level. if things get too easy then they might change.