Do I have to slide on a force play?

You're never required to slide, but under the force-play slide rule you can't interfere with the fielder:

• If you slide, slide on the ground and in a direct line between the two bases, or slide away from the fielder.
• If you don't slide, run to the side of the base, away from the fielder, so you don't contact them or alter the play.

If you go in standing up and contact the fielder, get hit by the throw, or alter the play in any way, you're out and the batter-runner is out too. The only time you can safely go in standing up is if no fielder is anywhere near the base.

At home plate, you may slide through the plate in a straight line without breaking this rule.