Hey there HiipFire. I'm a fellow rookie (been here since mid-February), but I do have quite a lot of experience with MM. Here's what I did when I first started speedrunning.
First off, choose the version you feel most comfortable with. For me, this was N64, but it may be different for you. I will say that N64 has the advantage of easy HESS (Hyper Extended Super Slide), but it's totally up to you.
Second, practice practice practice for hours before you even start an attempt. Look over the tricks in the run and practice them on a completed file over and over until you get at least a 1 in 3 consistency with them. If you dont know how to do a trick, most of them have a page on ZSR, others can be found on YouTube. Practice is key.
Third, watch previous runs of the route you're going to attempt. This will show you timesavers that may not be told in the route. After learning the timesavers, practice them as well. Timesavers are generally easier to master than tricks, but a few are just as difficult.
When you start the run, it is important to remain calm. Don't worry about the timer, just play through the route. If you're worrying about the timer, you'll start trying to do things too quickly. Believe me, I speak from experience. Speedrunning takes patience and control. If you're getting stressed, take a break and listen to some music or watch something funny to help you calm down. Stress will make you sloppy (again, I speak from experience).
Welcome to ZSR, man. I look forward to seeing your work