Cherries are grafted. You have an worthless undertook. Go to a nursery and buy a grafted tree.

If this specimen was in fact raised from seed then it is not a rootstock variety. If it was grown from a fruit you got off of a flowering cherry that you thought was nice then the way to get it on a single trunk is to take the tallest stem and tie it to a firm stake inserted near the center of the plant. (Without pushing the root crown down into the soil in the process). Keep that one stem tied in close to the stake, train it up the stake like a vine. Replacing the original stake along the way with a taller one if needed in order to get the trunk as tall as you want. Then let the shoot you have been leading up the stake start making side branches to form the permanent elevated top. (A production nursery would often cut the end of the staked stem off in order to get side branching immediately at that time).

The other stems coming from the base would be pruned off at some point once you got the stem you selected well on its way.

[Having just typed all this I see this is a 3 year old question]