Answer: I know it sounds barbaric, but pinching out the very tip of the stem will cause your plant to branch out. If you don't pinch, it will eventually branch out on its own, but it is liable to look rather thin and gaunt for quite some time before it develops branches. I've grown lots of avocado plants from seed. Wait until the stem develops two sets of leaves and then pinch the very tip of the growing point back with your fingernails (or use small scissors). The plant will then produce two new stems at the point where it was pinched. You don't need to actually cut the stem back, just pinch the point of the stem out.
Best wishes with your avocado!
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