Not sure how much experience you have, or how closely you've looked at this bike but:
If you don't know the history of the bike:
I'd change the oil and filter, check the valves, change the coolant, inspect the brake fluid.
Depending on whether the air filter is paper or foam, replace or inspect/clean/oil it.
Inspect the spark plugs, check the gap.
Inspect the chain/slack. If all is good, clean it up and lubricate it.
Check the tires for wear/damage.
Lubricate the clutch cable.
None of these are very likely to be your problem, but this will get you in a good place in terms of regular maintenance.
I believe there's two carburetors on the bike. You'll need to thoroughly clean them, inspect the diaphragms for holes, verify that the slide action is correct, and sync them.
