A hot bath with a handful of Epsom salts in it is often helpful for me when I'm aching and/or stiff.

And yeah, what muck_a_luck said. Don't be afraid to back off while your body adjusts.

FWIW, the delayed onset muscle soreness is usually much much worse when you first start a new exercise activity, and then things settle down quite rapidly.

But go easy on yourself tomorrow. Let the trainer know if you're still hurting. It might be a time to work on form and learning new things, rather than pushing yourself to the limit.