I used to grow every avocado pit I got when I lived in a snowy-winter climate, decades ago. The advice I recall reading (years later, perhaps) was to lop off the top when the seedling is small to force it to develop branches. I didn't do that, and I ended up with a bunch of sticks with foliage on top.
Assuming that this is not the one and only avocado you will try to grow, you could try pruning each one differently and see what result you like best.
If you don't like the lopsided look, you can force it to make a branch going in the other direction by lopping off the top. Cut just above a leaf node that's directly above your branch, making sure the leaf is pointing in the direction you want the branch to grow. New branches come from the leaf nodes, so you can control where the next branch comes from. Cut no closer than a quarter-inch above the leaf node.
Or if you want it to branch lower down on the stem, see if you can find leaf nodes lower on the stem.
(I know -- this is your first and you don't want to lop off all that lovely top growth.)
The sucker doesn't look vigorous enough to steal energy from the rest of the plant. You can leave it or prune it off, whatever you prefer.