I started with TPE Glitchless and I think it's a good starting point. It lets you focus on knowing the route really well and working on basics like mashing dialogue before jumping into the glitched categories.
it REALLY depends on what you want
what do you prefer, a highly technical speedrun with alot of glitches based around the punch card and skipping 50-90% of everything in waterfall onwards, or an easy to learn route that's often a little slow, and is very long?
TPE glitchless for sure. it's playing the game casually just a lot faster. Really the only thing you have to learn is quads for Asgore.
Neutral Glitchless (No Aborted Genocide) is easily the best beginner category. It's 30 - 40 minutes shorter than TPE Glitchless (while having basically the same route) and lets you practice bosses & advanced strats (Kindergarten Undyne & Essay Skip Skip) with a little extra HP.
https://www.speedrun.com/undertale/full_game#Neutral_Glitchless_No_Aborted_Genocide
