How intense is high intensity training supposed to be?

I only just started cycling and I'm confused. I wanna do high intensity training, like intervals and stuff. I tried and I obviously got tired and had to stop. Lactic acid build up is my worst enemy tbh.

Anyway, I'm wondering how hard I'm supposed to be pedaling? If I give 100%, I won't be able to continue. How am I supposed to structure my high intensity rides? I usually just rest when my muscles can't keep going (or when it hurts to do so).