So, I've lately been doing practice runs on PJ64, and I was wondering if there's a specific Emulator I need to use.

Most Emulators aren't allowed. So go for Project 64 v1.6 since all later versions are banned, you can find it in the resources tab.

Also, check this out: https://docs.google.com/document/d/1k02cyFMU2de1moy1NOUjX9M79I5N7tLEty7pSpenloE/edit

everything you need to know is in rules on the main leaderboards page