Welcome! You'll find a lot of information on this board.
Step 1: Locate the Factory Service manual on this board for your bike
here.Step 2: Read it through at least twice.
There should be about 3+ quarts of oil in your engine/trans. The best way to check it is to put it on the center stand, leave it sit for 20-30 minutes so all the oil drains down to the bottom, then unscrew the dipstick, wipe it, and then put it back in the oil fill hole. Do not screw it back in: let it rest on the top. Then pull it and read the level.
There's lots of opinions about oil for these bikes. You can't go wrong using strictly motorcycle oil from your local auto or MC shop. Some people use synthetic MC oil, some use synthetic diesel oil. If you DO use these, stay away from oils that say Energy Conserving. These have additives that will mess with the wet clutch, causing problems with shifting.
I hope this helps.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.