I would think the original finish is lacquer.
Once you get it stained, read the stain instructions as to how to proceed with finishing. After staining, some stains recommend light sanding, before finishing. It depends on what kind of stain you use, so read the instructions on the can of stain, to see what procedure is to be done. If you have questions about the instructions, ther should be a phone number, on the can, that you can call for technical assistance.
I assume the rest of the cabinet, other than the top, is in good shape and doesn't need staining or finishing. Before staining and finishing the top, you might want to drape off the lower cabinet, by taping newsprint around the cabinet, below the top. I use 2" painters blue tape for taping off areas, as this.
If you elect to finish with lacquer, I would suggest using aerosol spray cans of lacquer. Spray lacquer should be available at your local Home Depot, Lowes, or similar hardware/paint store. Other spray finishes are available, also, should you elect another type of finish. Read the instructions for spraying and practice your spraying technique on cardboard or scrap wood, i.e., the sweep of your hand/arm, distance from the surface, etc., in order to get a good smooth coverage coating. Lacquer and shellac dries fast, so take this into consideration, also. For any other finish you may use, follow the instructions as per that spraying.
As per your stain (oil based, water based, etc.), make sure your finish is compatible with the type of stain. This may be addressed within the stain can instructions. For a finish to adhere properly to the stain, light sanding may be required, or you may be able to apply the finish without sanding. Again, what are the instructions on the stain can, for prceeding with finishing. There may be specific instructions for different finishes. And again, if you still have questions, call the tech support, re: phone number on the can of stain, if need be.
Other than this (without seeing exactly what you have, there), I'm not sure what else to recommend or suggest. But probably most importantly, is to practice your spraying ing technique, if you spray, and practice your spraying technique, first, before committing to the cabinet's top.