Hiya Ulairi, welcome to the forums (and back to GURPS!)
If your group is kind of on the fence and not sure they actually want to play an ongoing GURPS campaign, I'd suggest starting with a very simple one-shot using pre-generated characters you make yourself, like it's a convention game.
"
Caravan to Ein Arris" is a free GURPS 4th adventure, you'll just need to make some PCs. I have a couple of
Quick-Play adventures for download as well -- they already have the characters but are a lot more free-form in terms of the actual adventure. Or, just go nuts and make your own adventure/PCs.
This has the benefit for the players that all they have to do is show up and give it a try. Once they have a great time and get hooked, then they can make their own characters for the "Sliders" campaign, or whatever you decide to play.
If the group is already on-board with joining a campaign (or is now on-board because they loved the one-shot), probably better (and more fun) for them to make their own characters. Depending on where they/you are in terms of the GURPS rules, this could be spending a few hours together making PCs from scratch with the Basic Set, whipping together some simple characters (using the "Pointless Slaying and Looting" article in
Pyramid 3-72 for Dungeon Fantasy characters, or my generic
Quick-Start rules... there are others), providing them with a variety of Templates to choose from ("
GURPS Template Toolkit 1 - Characters" is super helpful for creating your own Templates), or using some kind of
questionnaire to elicit responses from the players that you as GM can use to make characters for them.
Infinite Worlds games are super fun -- my favorite kind actually! -- and also play to GURPS' strengths, just be aware they can be a bit more work to GM simply because of the huge range of possibilities. You could probably get by with the IW chapter in the Basic Set, but the full book expands greatly on it and I really love it. :)
Regardless of what choices you make and what kind of game you play, I think you'll find these forums an invaluable resource. Everyone here
loves to answer GURPS questions and help new GMs!
Good luck on the game... I bet you all have a blast.
P.S.
The Fourth Edition "Discworld" has been completed but hasn't yet been scheduled for release. When it is, it'll be announced here.