You want to do nothing right now. Just plant it and let it establish properly.

If you were to remove one of the 2 major stems, it would leave the plant lopsided and also be too much of a stressor to remove so much of it. But trees can come in multistemmed forms as well :-)

Once it has become established, you can remove or clear out all the lowest branches, gradually working your way up the trunks until you achieve the canopy height you want.

How do I know when the tree is established?

Typically, for a large shrub or small tree of this size, you would allow at least 2-3 years, sometimes a bit more.

You don't need to be in a big hurry to prune.

why do you need to make a flowering shrub more tree like ....

when you can affirmatively state what or why you want to do something.. the actual pruning is easy ..

tree time is counted in decades.. shrub time about half that ...

so you plant will be established in 3 to 5 years.. and ther is no pruning until them.. if ever ....

and if you want a tree form in a hurry.. buy one already in that from.. regardless of price ...

Crepe myrtles have been bred to be many different sizes. That information should have been on the tag when you bought it. You wouldn't want to make the mistake of pruning lower limbs up until all you have is a topknot.

As said above above, Crepe are available is SOO many varieties, colors, sizes, hybrids. I was going to say plant it already, then realized we do not know where you are or your zone, or if you ever even want it in the ground. Definitely noticing more of them planted in the NYC/NJ Metroplex (which is now upgraded to a 7b). In NYC and on LI, you can see arborescent ones long established, give the surrounding suburbs a few more years of growth. Most people plant them expecting bushes, not trees, then are burdened with annual, heavy pruning. But there ARE cultivars that will be semi-dwarf or dwarf.. My experience with the super dwarf was the plant was a LOT less vigorous, and did not make it.

Above is a Dynamite Red which is pure Indica (indian), the Japanese hybrids are hardier if you are colder.

If you will leave the lower branches on then the trunks will become much stronger. Trimming them off too early causes the main stems to just whip in the wind. As stated, give it a couple of years before pruning. BTW, they are really fast growers especially if they happen to be the variety Natchez.

Is it a named cultivar, or a mystery crape? That would help you know more about what to expect, including how much space is needed. ( Not an issue if you’re going to plant it out in wide open sunny yard. )