For cycling:
Make sure your points of contact on the bike are comfortable, so hands, feet and arse. Well-fitting gloves, shoes and shorts/tights. As PP said, a bike fit is worth the money.
Learn how to fix a puncture and bring the appropriate tools with you (tube, tyre levers, C02 or pump). Anything else and you'll be phoning your broomwagon/taxi home.
Get to know your local roads, do you like hills or the flats. A cycling club will help with improving your pace and your library of good routes.
Be curious and have a goal. Want to see how good the café is in the next town, can you cycle 60 miles in four hours? Beat your own PB up your local hill.
Learn how to eat and drink on the bike without being a hazard to other road users. Strong core muscles help you get stuff out of your pockets without wobbling.
Get and learn how to use clipless pedals and cleated shoes. It's tricky but find a quiet road/park/car park and practice clipping in and out one foot at a time. Proper shoes and pedals make a big difference to your cycling efficiency.
Be confident in your own abilities, yes you might not be the strongest rider immediately but everyone has their own cycling strengths. Don't take any shit from anyone else.
Here endeth the lesson!