Cadence! If you feel like your bike is pulling you up big hills, you're doing it right. I can't be without a bike computer that tells you cadence. 90s on flats is doing well.
Carry a tube (or 2) and a pump and know how to change a flat.
Set up the bike to fit you. Start with the seat. Height, angle, front/back. Move in small amounts and start with a recommended start position.
Keep the chain lubed and do any other maintenance.
Be alert in traffic and ride on less traveled roads whenever possible.