Resources for Sci Fi tabletop game.

I'd just like to point out that I'm super new to roll20. But one of the reasons I joined is because I was looking to branch out into different genres of tabletop gameplay. One thing I'm really interested in sci fi based games like stars without numbers or starfinder etc. However I'm having troubling finding a comprehensive guide? A starter kit, if you will, that can introduce the basic rules and premises.  So if someone could just point me in the direction of those type of resources I would be really grateful! I'm looking forward to learning how to play in a cool sci fi setting! 

The best thing to do, within the parameters of Roll20, is to use the  Looking For Group tool and look for Science Fiction genre games that welcome new players. Roll20 itself has no resources to teach any particular system, or even rpg gaming as a hobby, but many of the games being run on the site are fine with guiding new folk into the hobby. When applying to a game, be forthright and honest about your relative lack of experience, and above all, be patient. There are many more people looking to play than are willing to run, and it can sometimes take time to find a group.