Beth,
Gayle's suggestions are good starters if your ancestor was in service in one of the Continental Regiments, but not so much for State Troops or County Militia units.
While the National Archives does have some scattered militia rosters, those records are mostly held locally. Some State Archives have also compiled County Militia records; this varies a lot by time and place.
State Troops' records are principally held by the States, but some rosters for some State Troops (such as NY Levies) can be found in National Archives.
The National Archives' holdings of muster- and pay-rolls are mainly within Micropublication #246, the rolls arranged very roughly alphabetically by State. These are the records from which clerks transcribed items onto old-style file cards many decades ago. If you write the Archives for a "service record" you will get photocopies of these cards. They are not complete transcripts of what's on the rosters; many names are not correctly transcribed and not all entries were transferred to the cards.
There is another huge body of Revolutionary War records in the Archives that is also on microfilm, but it is seldom used because poorly indexed and the microfilm indexes are hard to use. It's great if you know what's there and how it is indexed.
You can borrow the actual microfilm by inter-library loan for a few dollars per microfilm roll. You can do the same through a local LDS
Stake.
Heritage Quest's offering of pension files is only the Selected
Records from Micropublication #804. What you want to look at is the complete file in #805. Here again you can borrow the microfilm as above. The rolls are arranged alphabetically by name of soldier. More libraries have the listing of what each roll covers than have the listing of the rosters' arrangements in Micropub. #246. Many major libraries have the pension application files in their collections already.
Good hunting,
Judy