Good, and difficult question.
For the board, you need to decide what size to get, both in terms of length and width. Both are big topics of discussion with no single correct answer for everyone.
Board length: A bigger section on length can be found in section 1.07.03.
Board width: A bigger section on width can be found in section 1.07.01.
Read the sub-sections for more info all round.
Brands
There are now so many good makers of boards and such good information available on the internet, then if you do a bit of research, you're not going to buy a duff board.
There are lots of big names in snowboarding, some have been there from the beginning, some crossed over from skiing as the market dictated they should. Names like Burton, Ride, K2, Salomon, etc. There are also some small custom board makers that make very good boards (e.g. Donek, Prior) and shouldn't be discounted just because you've never heard of them. Do some research. Of course, the $100 boards from your local mall aren't going to be top of the range, but may be adequate enough to ride during the learning curve.
Style
Buy a freeriding board, soft boots, and soft bindings.
Setup
Work out some starting angles like this: Stand up on the floor with your feet about 20"/50cm apart feet slightly apart. Bend your knees so you're squat down, let your feet do what they want. Stand up again and do a few gentle squats. That puts your feet into a natural position. Now somehow you need to measure the angular difference between your feet. Usually it's about 30 degrees, but could be anything - as long as it's comfortable. Then transfer this to the board.
Set your bindings to 20"/50cm wide, 1" back from center, 30 degrees on the front, then set the back binding to be 30 degrees minus the difference you measured above. If your feet were 30 degrees, then set the back to 0. If 20 degrees, set your back binding to 10 (or the nearest available setting).
Now, go learn to ride, then play with the stance to see what works for you. If your knees or feet are hurting because the position (and not because they're unused to exercise), tweak the settings by amount and try again.
New or Used
You can save a bundle with a used board. Buy one that isn't too old and has either the straight 4x4, 2x2 or 3d binding inserts, isn't too beat up (the base and edges look ok), and hasn't been pounded to death (it still has camber).
Learning to ride
Take a lesson. Even if your friend is "da bomb", a qualified instructor
knows how to teach beginners, and should have the experience and patience
to do so. You don't really want to burden a buddy with being stuck with
you on the bunny slope when (s)he could be off having fun at the top of
the mountain, plus you won't fall out over it.