Newbie looking to finish Walnut!

For what you are doing I would use a pastewood grain filler and fill the open grain of the wood. It's available by mail order from Mohawk Finishing Products in walnut color. Sherwin Williams also makes a good grain filler but it's natural in color. You can ask them at Sherwin Williams to tint it for you. It can be colored with the same tint that is in their machines to mix paint. After you fill the grain you and the filler dries you can sand the wood with 180 to 220 grit paper to get the excess of the grain filler off the surface and it shouldn't alter the color very much. If you don't have the means of spraying you might finish the piece with Sherwin Williams interior oil based polyurethane. I would start with a gloss sheen until you get to the last coat and then put either semi-gloss or satin on. The flattening agents in semi-gloss or satin cloud the finish somewhat and it's better to build the emulsion with gloss which is clear. If you have the means of spraying you might consider using a cab-acrylic lacquer. It's not as tough of a finish as the polyurethane but your chances of having a nice professional looking finish are so much the better and is much easier to do. You can use it alone or it makes it easier if you first seal the wood with a vinyl sealer first. Finish the same way, start with gloss and finish with semi-gloss or satin.

If you are a complete newbie at finishing I wouldn't start with a pore filled finish. I think a wipe on polyurethane finish either oil or water based would be fine. For an example lets assume it is oil based poly or rock hard table top varnish, just thin the product down about 50% with mineral spirits or VM&P naptha, wipe everything with a rag saturated with the thinned finish (wear rubber gloves of course) and then wipe completely off with a clean dry cotton rag (like a white t-shirt) Let it dry for a day, scuff sand with 320 silicone carbide sandpaper (the white kind, not the black wet or dry) and repeat the process. Do this everyday until you're happy with it. For added protection after the final coat apply some clear paste wax using steel wool or scotch brite as an applicator (use the pad to get the wax out of the can) and then polish with another white t-shirt. MAKE SURE AND TRY THE ENTIRE PROCESS ON A SAMPLE BOARD BEFORE YOU TOUCH YOUR PROJECT!