Basic stuff (installing memory or drives.)
Screw drivers, - get one of those pro regular + jewlers sets. and you'll be fairly sorted for desktops and laptops.
Anti static wrist strap + mat
a boat load of USB sticks that you can install tools onto. (ideally use a single USB stick for each job, and then wipe the stick and put the tools back on it at the end of the day, - you don't want to have a virus put on your tools stick by customer 1 that you then stick into every other PC for the day...
on a similar vein, you'll want recovery CDs for all operating systems since XP.
Live CDs for some Linux distros,
Data recovery tools.
External drive caddy (so you can take data off of a clients hard drive if their main board has failed or something like that.)
then a premesis, place to work.
and public liability insurance...
if you want to go into hardware repair, a hot air soldering station, microscope, and buckets full of partially failed devices that you'll pull components off when needed.