Being new is always fun, the SMS community and speedrun community in general are very welcoming to new players who are half serious about speedrunning. You'll be surprised at how fast you get pulled in :) The best thing to do is to watch livestreams on twitch of sunshine runs and ask questions galore. Most people are always willing to give advice. Just follow the super mario sunshine directory on twitch and talk to people who are speedrunning live.

Before you actually start streaming and speedrunning yourself, I would recommend practicing for a few weeks (or even months if you're someone like me). You'll probably just want to play the game casually to get a feel for the controls, and then start doing the strats you see. There is also a full guide being made to see all the strats done properly. Most of the tricks you see will take a while to get, but the goal is not give up and do easy strats. This applies to the skips as well, gelato skip and the others will seem impossible at first, but giving up gets you nowhere.

Also, when you learn, take it one step at a time. Don't learn the whole game in a few days. I didn't start learning the last world of the speedrun until about 3 months in. I only progressed when I was satisfied with the last world, and it paid off. Others may have different approaches and opinions to this but I just don't think it's worth doing runs until you thoroughly know the game, especially for one as competitive as sms.

Anyways, I hope this helped and good luck! I would be glad to answer any questions :)