I was going to suggest watching the presentation from FIRST Championship on staying involved with FIRSt programs in college, but it appears those conferences (despite being taped) never made it up. If someone knows where those are, it would be great to see them, because there were some truly fantastic presentations.
Both GOFIRST (my organization) and Purdue FIRST are older organizations, and I'm not sure how helpful anyone from those groups will necessarily be when it comes to advice for starting a group. I would definitely recommend getting in contact with
FAMNM, since they are a more recent addition to a growing group of FIRST Alumni organizations (FAOs).
That being said, here's my advice for getting started, but take it with a grain of salt because I never started a student group, I only joined one.
I would follow a similar path as Sravan suggested. You first need to find those alumni or people who are interested in robotics/volunteering. Once you find those people, you need to find out what they want, and what they think would be valuable goals for the group. Some FAOs are solely dedicated to volunteering-- GOFIRST started this way, while others also mentor teams, host events, or participate in collegiate level competitions. GOFIRST does a mixture of all of these things, but it might make sense (just like with robots) to focus on doing what your group is really passionate about first.
Once you have an informal group, you'll want to figure out who to contact at your school to help you get through the necessary paperwork and find funding (assuming you need that). You'll want to get really familiar with your schools rules regarding student groups, and someone on the inside is the best person to do that. From there, it's probably going to be more context-dependent.
Best of luck, and feel free to reach out to me via PM or email if you have any further questions!