I planted a apple seed about 6 months ago, and I have a little tree now.
It's about a foot and a half tall. I have it in a pot on a window sill right
now. However the only growth on the tree is up. It is not branching out at
all, just some leaves on the trunk of the tree. Is this normal?

Yes, for a first year seedling. What are you planning to do with the tree?
Remember, it may not give fruit that is worth anything.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)