HELP Please another NEWBIE stupid question

Since you don't say what bike, I'll assume it is the 125 SX you have listed. For that, simply refill the gearbox with the recommended quantity of the recommended oil. The quantity is normally stamped near the oil fill on the case. The only ones with separate gearbox and engine oil, are some of the newer 4 strokes. The engine on a 2 stroke has the lube supplied in the premixed fuel, or by an oil injector in some cases.

To change the wheel bearings, you support the wheel on some blocks of wood, and drive the bearings out with a hammer and a drift rod. Sounds kind of crude, but that how it is done. To seat the new bearings, gently tap them into the hub with a socket or bearing driver the same size as the outer race of the bearing. Be sure to grease the bearings and spacer as you are assembling it, to provide lube to the bearings. Then install the seal with the outer bearings well greased.