Since you just got it, you should change all the fluids so you know it's been serviced and has the right stuff.
Front hubs: demand drive
Frond diff: angle drive
Engine: 2 qts PS4 or mobil 1 0w-40, Bosch 3323 filter (the oil is about $20 for a 5qt jug at walmart, the filter is about $6 at auto zone)
A wix no-toil air filter is about $20 at oreillys. I don't have the part number, but it's easy to look up on the wix website.
As far as service interval, annually should be fine for you, unless you ride in very dirty, dusty, muddy conditions, or if you take it swimming.
2006 Sportsman 450
27" Vampires on ITP Delta wheels
3 headlight mod
55w driving lights