As Jaffa says, the manual
is a very good investment,
other than that a lot depends on 1) how mechanically competent you are, 2) whether it's been used a) for racing and maintained to any level or b) thrashed round fields and tips and then thrown in the shed.
It is a race bike, and needs the same level of maintenance as road race tackle really, but kids get hold of them pretty cheap and then rag them to death.
A big part of the maintenance is to keep the airfilter cleaned and reoiled regularly (take it off, wash with correct solvent/cleaner, dry, reoil, refit) change spark plug reasonably often, handy to have spare one if plug oils or wets up with fuel. Use good quality two stroke oil for race/motocross engines, competiton grade, super unleaded fuel is a bonus, you can add a little bit of octane booster if needed. Keep throttle and clutch cables and kill switch maintained as you don't want to be caught out by 40 odd horsepower on low gearing refusing to be shut off!! brake pads and to a lesser extent discs will wear a bit as will chain and sprockets if you are riding in a lot of wet muck or sand regularly, keep eye on them.
Suspension is generally serviced about ever six months when raced regularly , or if it starts leaking, it does travel a fair distance up and down, maybe as much as the bike travels forward!
Pistons are replaced at a set number of hours and piston rings prob twice as often (gets more frequent if you don't maintain airfilter/ use crap oil. not much gear oil in there so usually changed every few meetings especially if you slip the clutch a lot. Crankshaft and bearings last all season usually.
You will have a power valve in the exhaust, some years were more durable than others, if this gets worn excessively it may touch the piston, causing damage
Hope none of this is too alarming!! at least you had the sense to ask! any more questions and I'll try to answer them, also most motocross/off road shops are run by knowledgeable enthusiasts who will answer your queries/try to help if they can, so ring or call into a few, would help to know roughly were you are if you need any recommendations who to go to.
Above all, enjoy, have fun, and give it a go!