It you are talking a TUNE UP only, the NCCheesehead is correct .... it basically consist of only the three items he lists. Add the blade sharpening and you are done.
Buy a new plug ... check the gap, and install. Piece of cake.
Oil change ... buy one or two quarts of oil. Depending on the engine brand, sometimes draining the old motor oil out of a mower can be a real mess so be prepared! Add the appropriate quantity of new oil and you're set. Again, overall a pretty easy task.
Take a look at your air filter element. Sometimes they are a multi-fold paper design ... if so, buy a new one and stick it in. Sometimes they are a foam element that can be reused. Wash out the foam element with soap and water, rinse well, squeeze out the water and saturate the foam element with new engine oil. Squeeze out the excess oil and install the element. Again, just messier than difficult.
Sharpen blade. You will need either a bench grinder or a 4-1/2 hand-held side grinder. REMOVE THE SPARK PLUG WIRE FROM THE PLUG BEFORE YOU BEGIN !!! Tilt the mower back (actually, this blade work is best done after you have drained the old motor oil, but before you have install the new oil). WEAR GOOD LEATHER GLOVES !!! Hold the blade with one hand and remove the blade retaining bolt on the bottom end of the engine shaft under the mower deck. Sometimes this bolt can be tight. Make sure you keep track of any washers that may be with the bolt and blade. WEAR SAFETY GLASSES !!! Gently grind the cutting edges on the blade being careful to not "burn" the blade; ie., keep your grinding rate slow so as not to overheat the blade with the grinder. Try to remove essentially the same amount of metal from each end of the blade to keep the blade balanced. Reinstall the blade and any washers being very careful with the SHARP blade. Snug down the retaining bolt and you're done. Again, not difficult, but you do need a grinder, safety glasses and gloves.
Good luck!