please read the thread
Please read this:
Sticky: FORUM RULES - READ THIS BEFORE POSTING
before posting.
You haven't given us enough information to help you. The thread above gives you details about what to put into your post so we can help.
It also points out the handy forum search feature. There are thousands of posts made by hundreds of people here. Chances are excellent someone has already posted your question - and your answer.
If you are talking about the whole screen being "cut off" then you probably are using a PAL ROM - for example "E" (Europe) instead of "U" (United States). The latter would be an NTSC ROM. This can be fixed by tweaking the RDB file, but you might just be using an old version of the RDB or of PJ64 - you haven't told us anything about that you see, so it is hard to guess.
Good luck, and have fun.