Why do most of the foods I bake on the cookie sheet end up burning on the bottom?!

Is yoru oven preheated when you put the cookies in? it should be.

Is yoru oven rack in the center of the oven? should be

Is your pan dark-colored? Try using parchment paper on top of it, or getting a shinier, lighter-colored pan.

Make sure your oven works properly. If it is elec, is the top element coming on? It may be out, and the oven is having to heat from the bottom only, which can scorch food.

To me, refrigerated pillsbury dough burns easily on the bottom. Use a scartch dough.

I'd just add that most ovens have some hot spots or hot sides, and that it's also possible that your rack positions still allows the bottom of your baking sheets to be too close to he bottom coils. Bying one of those cookie sheets that has two layers with air in-between as insulation can help in that situation, or just elevating the baking sheet you have on some balls/logs of aluminum foil, a couple of pieces of wood, etc.

It's also possible that your oven is just too hot and reaching higher temps even if the dial is set at a lower temp (get an oven thermometer).

if you have the rack set at the very bottom, that could be reason #1.
If you are baking things too long, that could be reason #2.
Oven thermometer no longer accurate is reason #3.
Experiment a little.
Your first line of defense when something burns is you left it in there too long or the heat was too high. Next is placement. Most things bake best in the center of the oven.

Are you cookie sheets dark? If so, your oven should be set 25 degrees cooler than your recipe calls for.

Is your oven accurate? Use an oven thermometer to check the accuracy of your oven

Use Parchment Paper. It will absolutely stop the burning bottoms. I use it all the time. Plus it has the added benefit of making clean up easier