If you want to get fruit from most types of apple, cherry, pear, etc. trees you have to buy 2 varieties that can pollinate each other. If you only want 1 tree, you can graft short sections of limbs called "Scions" from say, 1 sweet cherry like" Bing" to another like "Black tartarian" Check internet sources in your state or similar climate areas for compatible pollinizers -they have to bloom at the same time. Supposedly, sour and sweet cherries do not commonly cross each other effectively in most areas.This will allow the tree to pollinate itself.
If explaining this in a high school, wait for jokes to subside....
Basically, you select a twig piece (scion) with 3-4 buds and make 2 diagonal cut below a bud to get a pointy wedge. A diagonal cut on the "graftee" (forget tech. term) should be done on a slightly thicker twig, above 1 bud and slightly below another. This opens a spot to place your scion. A sharp cut straight down is done, you slightly twist the knife, and jam in the scion. The side with more bark gets to face out. You are trying to get the greener outer layers of the 2 twigs (cambium) to match up and join together as they heal. Tape it tightly with grafting tape, or electrical and/or wife's "nurse tape" if you can't find any and forgot to order it