Been drying my own vegetables and some fruits for years; own three dehydrators. Some of the most common ones are:
Apples: Peel, core, cut into slices.
If you don't want them to turn real brown, soak them first in water with lemon juice or pinapple juice; or buy one of the anti-oxidizers at the grocery store. They're done when they are still a little pliable(4-18 hours depending on your dehydrator.
Tomatoes: cherry or paste(italian) tomatoes work best. Just cut in half and place cut side up on the trays. Dry until leathery May take from 12 to 36 hours depending on the tomatoes.
Bell peppers: cut out the stem end,
cut in half, remove the "ribs" if you want, then cut into large chunks or slices(they really shrink). Dry until leathery.
Hot peppers(jalapenos etc). Leave whole but use a sharp knife to pierce the peppers in a couple of places(to allow moisture to escape). This is for thick peppers like jalapenos. For the thin ones(cayenne etc) just place them whole. Dry until leathry.
Onions: Peel and cut into thick(1/4") rings, separate the rings and dry until brittle.
Beets: steam, peel, slice or dice. Dry until brittle.
String Beans (leather britches): Cut into 1" peices, steam/boil until tender. Dry until brittle. An acquired taste(taste terrible to me).
Bananas: Remove the peel(obviously), cut into about 1/8" slices. Dry until crisp.
Cranberries: Heat in boiling water for 1 or 2 minutes. Spread on trays. Dry until slightly pliable.
Great for baking.
Celery: Slice the stalks into about 1/4" pieces. Dry until brittle.
Grapes(raisins): If they're from the store, they've probably been waxed, so bath them in hot water for a couple of minutes to remove the wax. Dry until pliable. Can take up to 36 hours.
Peas: Shell, blanch until tender and spread on tray. If the peas are smaller than the holes in your trays, use cheese cloth and lay the peas on that. Stir once and a while while drying. Dry until brittle.
Mushrooms: slice or cheat like I do and buy them already sliced at the store. Dry until leathery.
These are the ones I've tryed over the years. One thing I don't recomend drying is parsley, Tastes like dead grass. Freeze it instead.
When I say leathery I mean DRY and pliable. If you're not sure about something it's better to leave it in longer, even if it gets brittle. There's a learning curve involved here. Also, if you're using a dyhrator that uses bottom heat, you need to rotate the trays every so often(there's more heat at the bottom) to keep things drying evenly. Another thing is some pieces will dry faster than the rest, so it's good to check each tray individually and remove those peices/chunks/slices that have already dried. Have fun.