How do I cum?

Rub it longer and you'll cum. Just keep doing it and use a vibrator on it until you cum.

Find porn that you like and watch and play with yourself and experiment.
Sexual stimulation is largely in the mind. Try it.

You have to experiment. I've known some women who can only cum from clitoral stimulation, and some who can only cum from vaginal stimulation, or a combination of vaginal and clitoral. My current wife hates clitoral stimulation and cums mostly from vaginal penetration, but also can cum from nipple stimulation. She cums best when she's on top riding me while I suck on her nipples. Some women supposedly cum from anal stimulation, but I've never met one. You might play around with that too.

A vibrator of significant girth. You may not cum from clitoral stimulus so you like will from penetration. Girth is more important than length in a dildo.

Perhaps you need someone to physically help you