It depends a lot on which Fallout.
I'm going to assume you mean Fallout: New Vegas since it's the most recent. In that case...
I usually tag Speech, Science, and Lockpicking. That allows me get through most early-game conversations, access most early-game computers, and bypass most early-game locks.
I'd say there's a good argument to making Repair a tag skill, since it'll let you successfully complete the Repair skill checks and give you access to the Light Touch perk (gives you +5% crit chance and makes you practically immune to being hit by crits when wearing light armor) at level 2... but I don't generally do it because having Science usually gives you an opportunity to lower the Repair skill needed to pass a Repair skill check, and the early-game combat is easy enough that Light Touch (along with a tag skill for a weapon class) isn't really necessary.
For the other Fallout games, these would have been my choices...
Fallout: Small Guns, Speech, Lockpicking
Fallout 2: Small Guns, Speech, Stealing
Fallout Tactics: Small Guns, Energy Weapons, First Aid (I would have other squadmates take care of things like Lockpicking, Repair, Doctor, etc.)
Fallout Brotherhood of Steel: Bwahahaha... "skills"
Fallout 3: Small Guns, Lockpicking, Science
You could probably drop the weapon skills (well, except for with Tactics) and take something else instead. I just like having the early-game edge. And even then, you could probably make due with just about any weapon type (well, except maybe Throwing...), I just like Small Guns.