Metered head or plain-prism? Round or square eyepiece?
If it's the metered head (like the FTn) or the plain prism with round eyepiece, it will take the same screw-in diopter as the F2, FE/FM series, F3 (non-HP), Nikkormats and most other manual-focus bodies. The thread diameter on these is about 19mm.
If you have a plain prism with rectangular eyepiece, you'll need one of the old square diopters that slides over the eyepiece, or one of the eyecup adapters that is threaded to accept the round screw-in diopter.
I've been looking for one of the rectangular diopters, or the eyecup adapter for months, and have only found one on eBay that they wanted $50 for. I'll keep looking...
I don't know how bad your eyes are but if you can get an exam and new pair of glasses you may not need a diopter. Case in point is me. I had to get a +0.5 diopter for my F2A as I just couldn't see well enough to focus with the standard diopter. After exchanging the diopter it was like magic. I had to do the same for my M2.
Several months later I noticed even with the glasses on I couldn't discern things at a distance and decided time to go to the ophthalmologist. Long story short I had to get new glasses(I'm near sighted) and he told me NOT to order any diopter until I tried out my glasses first. I ended up having to take out the diopters I had bought and go back to the original ones and could see perfect with them.