There is a lot of clothing designed just for snowboarding. It tends to be reinforced in the knees, butt, shoulders, elbows, palms and fingers. Some clothing is even padded in the stress areas with foam or plastic.
Considerations here should include these facts: a beginner spends a lot of time on his/her knees and butt, snowboarding will wear out a cheap pair of gloves in a few days due to the abuse, because of the bending down/sitting/falling, the clothes should not be restrictive, and the pants should be waterproof - use something like Nikwax on old clothing to give it back some waterproof qualities.
For the first week on the hill though, it isn't necessary to spend 100s of dollars/pounds/euros on new clothing. You can borrow from friends who board or ski, even if it is old. I personally learnt wearing a couple of layers of stuff, topped with a waterproof one piece black over-suit which I had for riding my motorbike in the rain. It kept me dry. My biking gloves though, whilst being reasonably warm, were destroyed after a week of getting frozen and shredded up on the hard packed groomers. My buddy borrowed a multi-coloured two piece ski suit from a family member... back in those days it wasn't out of place though.
Clothing's Golden Rule: do not wear cotton - particularly for a base layer. It soaks up moisture and keeps it there, taking ages to dry. You can then get hypothermia. For your base layer wear something designed for the task, something that is breathable, wicks away the moisture and dries quickly. Any good outdoor clothing, snowboard, ski, mountaineering shop should carry this stuff.
Helmets keep getting more and more popular, and not only for those that
are pushing big tricks. However, this is not a place to lecture you about
the safety of your head. If you think you should wear one, then do so.
You won't look out of place - far from it.