A good helmet is your No1 priority and as has been said try on a number till you find one that fits snugly.

I would say that getting a good set of boots and gloves is No2 in terms of spending serious money to get a quality product.

As for jacket and jeans you need protection but I wouldn't spend a lot at the outset - get something serviceable till you know what you want - whether leather or fabric, custom fit or off the shelf. I don't know what the weather's like where you live but if you can stand wearing leather then it offers the best protection. Look for CE marks and as has been said get armour on knees, hips, elbows, shoulders and spine.

hope this helps