Form = pedal in circles, don't just mash down each pedal. Pretend you're scraping gum off of your shoe at the bottom stroke, pull back (uses your hamstrings) crest the top, push forward, scrape gum, pull back..rinse & repeat.

Know that your power comes from your trunk. Envision your stroke originating from your hip, not the knee or ankle.

Keep your upper body loose. Don't grip the handlebars tightly, unless you need to hang on over rough terrain - and even then - watch that death grip. Keep a nice bend in your elbows - practice keeping your elbows soft. You don't need to expend extra energy in tensing up your upper body.

Suck in those abs to protect that lower back.

Do not (repeat) do not push big gears in the beginning. Keep your cadence between 90 -110 strokes per minute. Even going uphill and in the wind - use those shifters! Your tendons & ligaments do not develop as quickly as your quad muscles & hamstrings. You don't want knee problems from the start. A cadence feature on your cyclometer is a very nice thing to have.