Isis,

Or we could just switch trees lol. I have a black mulberry here that insists it wants to be a tree even though I want it to be a bush. If it has multiple stems coming from the ground you will have to cut the extras and just choose one to be the trunk.

Choose the most vertical branch and remove the others, either completely, in very early spring, or gradually, with a lot of summer pruning. If needed, use a piece of rebar or other stake to keep the chosen branch straight and vertical and remove all branches more than a third the diameter of the trunk (that is branches coming off your chosen branch) at point of attachment to maintain vertical growth (apical dominance, which you can look up).

You may want to look at see if the nursery provided you a grafted mulberry tree.
If it has stems coming up from the ground it may be from the mulberry they used as a rootstock which I think I read is usually bad (like Russian mulberry rootstock I think?). In other words, if its grafted, get rid of those other stems coming up from the ground anyway. and keep the main trunk/stem that the plant came with.

If its not, then just pick strongest growing one as the main trunk (and may want to train/tie it upwards along a wood stake so it grows very straight, and get rid branches you want later on [I forget when to prune, since mulberries 'bleed' white stuff. You can google it.].)

-Ari