Did you repot it when you got it? If so I wouldn't prune it all yet. Let it settle into its new home. After that you can trim a few inches off above a node. Not so much pruning as harvesting.

Yes I repotted it when I got it. How long should I wait before pruning? And would I just make a cut at that horizontal red line?

Although it's about 10" tall but I usually prune it when the stem or branch has hardened a little bit, when it turns brownish.

After that it will look like it isn't growing for a while but then it will eventually start branching off.

But where on the rosemary plant do I prune it, the red horizontal line?

As earlier posters have stated, with rosemary it's really not pruning as much as harvesting when wanted. The last poster was very accurate--a good place to snip is where it starts to get woody. It will encourage bushing, but with such a baby plant you can't expect a large harvest right away.