Game Maker has free trial editions, but it's difficult to use for complex projects like RPGs if you're a beginning user.
If you're willing to discard graphics and learn some coding, you can download the Python interepreter
and create an RPG as simple or complex as you want- however, the lack of graphics means that you would have to describe everything in the game through your writing.
No matter what you do, it's going to help infinitely if you have a particular role-playing system in mind; for example, Planescape: The Torment
and Baldur's Gate
are based on the Dungeons & Dragons rules, and the Knights of the Old Republic
games are based on a modified version of D&D.
Since it's a sci-fi game, I'd recommend the Star Frontiers
system (under "Digitally Remastered" on the left)- it has rules for space combat in the Knight Hawks supplement, and for mutation in the Gamma Dawn book. The rules from Zebulon's Guide to the Frontier
(an update of the core rulebook) are incompatible with the other supplements IIRC.