What brand and model is the fork? Check the manufacturer of the fork's website.

I'm assuming that you have a carbon fiber steerer. If so you definitely want a compression plug, not a star nut.

You also need to know the size of the steerer tube, which is likely 1 1/8"

You should be good with any compression plug designed to be used with a CF steerer. I would check with the fork manufacturer however just out of caution, because some have specific requirements (such as a glued in reinforcement sleeve on alpha forks).