I purchased an electric heater for my garage that requires 240V and about 25
amps. I'm looking for a little advice on how to wire up an outlet to go
with it.

Right now I have a box with the wiring pretty much ready to be routed out.
The main wiring from the house has black, white and red, with the black and
white each going through a 40 amp fuse. The outlet I need says NEMA 6-30,
it's rated for 30 amps and has two horizontal prongs plus ground. I believe
the correct way to wire it is with the black and red to the horizontal
prongs (the ground is just for safety). My question is about the ground.
There is no ground currently wired into the garage, should I connect the
white wire, leave it disconnected, or what? Is there some relatively easy
way to make a good ground? I wouldn't mind switching over my 110 outlets to
3-prong as well.

Please fill in some details.
Is the garage detached?
What wiring method is used to supply the garage? i.e. rigid metallic
conduit, triplex, rigid nonmetallic (PVC).
Is there a bare or green insulated wire run with the black red and white?
Is the box you already have in the garage a junction box or a panel
cabinet?

I will need this information to provide you with good answers to your
questions.
--
Tom