Welcome to the club! I've been back on mountain bike twice now. I've been doing club rides so I have people coaching me as I go - really nice.
Things I've learned so far - look only where you want to go. Keep your weight back all the time. When doing a steep ascent I tend to pull my front tire up so - butt back, back flat and get down close to the handlebars seems to work for me.
And remember it's about having fun.
The really experienced folks will tell you more I'm sure.
Veronica