Within the last year I recently switched my gaming group from D&D 3.5 to GURPS Fourth Edition. I can give you some ideas what I did:
1. I wrote out a list of Combat Options that included all the things I felt would be useful to the first few sessions: that is, I went through
Basic: Campaigns rules and put together a bullet-point list like this:
- All-Out Attack. Blah blah.
- Move. Blah blah blah.
- Move and Attack. Etc.
I did
not include the things in the Campaigns book which we wouldn't be using: in particular, high rate-of-fire weapons (like machine guns), vehicles (no mounted combat), no rules on Fatigue (at least at first), no rules on Extra Effort (at first).
2. I wrote a short page explaining the 3d6 bell curve, Critical Success, Critical Failure, and what a Quick Contest is.
3. I wrote up a brief Combat Flowchart so they'd see how things happened and in what order. My combat flowchart included page numbers I could consult in the Campaigns book for further detail.
4. Writing out a summary of the rules helped
me to understand them more fully; giving a copy to the players helped
them to understand their basic options. Over time we have added more rules and more complexity.
As far as what to run, I started with superheroes. In retrospect I might have started a bit more simply, but heck, I recommend you start with something that you'll find entertaining enough to stick with. :)