Hello and welcome bradscott19!
If the stamps are not sorted into countries yet then that is your first thing to do (if they are loose).
A stamp's value is most of the time deondant upon it's overall condition. Mint (new, post office fresh, with gum usually) is usually worth more than used (cancelled by some postmark or obliteration, postally used).
After that, the colour (bright, dulled, sun faded, chemical accidents, scotch tape stuck to it), paper type (for older stamps), rips, tears, thins, creases (all bad), whether it has been hinged (little glassine paper hinges to stick a stamp onto a paper album page) for mint stamps, the centering of the design within the whole of the stamp so no perforations (wiggly bits around the edge of a stamp) are sticking into the design), the actaul perfs (perforations) around the stamps, whether they are all intact or bent, creased, missing, pulled, cut close, mold (alos known as rust or toning) affecting the stamp from poor storage in humid and dark conditions, the cancellation (postmark) itself, some make stamps more valuable to a cancel collector (like me) if they have the place name and date showing especially.
Old stamps are usually just that, old. They printed millions of these stamps usually. If they are the common ones used by evberyone to mail or post a letter at that time (depending on the postal rates in effect). So not all old stamps are valuable.
A catalogue in the reference section of your library (or some can be borrowed also) is a rough guide to how old a stamp is and its value if in really great condition (see above) and if someone wants that stamp. Not everyone wants every stamp. There are now way too many stamps being made. Some people just want newer stamps, some just want older ones, or just with certain topics on them like cats or digs, bridges, airplanes, boats, trains, etc. Or really pretty ones like butterflies or flowers. Or made by engraving types like Sweden stamps mostly and older stamps from everywhere.
If you can post a few pictures or scans (best) on a new topic started on SCF here then people will be glad to help you figure out what they are (country issued, year, value, whatever). Too many posted at once will make people tired and uninterested. A few at a time to help you figure out what you have and the rest of your stamps.
If you just want to sell them then think first that stamps are the #1 hobby in the world, lots of history, beauty and enjoyable time spent relaxing with stamps if you have the time or are inclined. After that, the more organized they are the better chance you will have of getting a fair price (not catalogue price, that is what you buy them for from a stamp dealer), maybe 25% of that if lucky.
ebay is fun but takes time too. It comes down with anything the balance of time and money.
Sometimes stamps are things you can study for a lifetime of enjoyment. Take it slow or at your own pace as it is Your thing you are doing.
Don't throw them out if you are tired of the whole idea. Give them to Scouting, Veterans, Churches, charities, etc so they can use them or perhaps sell them.