Great! Chinese elm is one of the few species of trees that can be treated as either a deciduous tree (if you keep it outside) or a tropical tree (if you keep it indoors all year).
Since you are keeping it indoors, we will provide you with care instructions as a tropical tree. First, as everyone is pointing out, getting enough light will be your primary challenge. With enough light, the tree's foliage will be lush, compact, and dark green. If the tree does not get enough light, the foliage will be thin, leggy, and light green. Your tree is showing signs that it is not getting enough light. Try to put it in the brightest possible window, or else buy a plant light to supplement whatever lighting it is getting.
When you water your tree, consider putting it in a sink with a spray attachment - or even occasionally putting it in a shower - so that the tree gets rinsed completely and the leaves get wet.
Because your tree is going to be a tropical, it will grow all year, so we want to prune it all year - any time the tree's growth starts to extend beyond the outline of the shape of the tree you want to maintain. Right now, there are many branches that are growing too long, so you will want to trim those back to the first two leaves (if the branch is strong) or perhaps three or four leaves if the branch is weaker. With Chinese elms, the new growth will always push in the direction of the last bud on the branch (when you prune). So if you want your branch to go left, leave the last leaf on the left. If you want it to grow right, leave the last leaf on the right. Finally, eliminate all growth that points straight up or straight down.
This should be enough to get you started!
