Have fun. That's the obvious bit that most seem to miss when getting a bike for health reasons of to 'get in shape' but if you don't keep it fun then you'll most likely stop riding.
Learn how to do basic stuff - fix flats, change inner tubes, figure out before you have a problem on the road how to get your wheels out to fix a flat tire. You might remember how to do this from when you were a kid but I've seen some folks prepared to walk 10 miles home or call for a ride... Personally, I carry two spare tubes and a small puncture repair kit and two tire levers. I also carry a pump rather than CO2.
Ride often but don't get into the trap where you feel like you HAVE to ride every day. If you feel tired then chances are you really are and taking a day off or two might not be a bad thing.
If anything hurts when you ride - find out why. If you're riding casually then nothing should hurt. Only if you're riding at your limit for a short duration should things get uncomfortable because of the effort.
Stretch.
Read the posts in the Clydesdale forum. When I used to race back in the day I was mid 140lbs. When I got back on the bike I was closer to 230... Bigger guys have to deal with issues that the waif like guys don't have too.
Vanity. Screw vanity. If you're somewhere hot - get a top with a good full length zipper and if you need to unzip it, unzip it. Fat is a great insulator... this means that when you get hot from riding you'll have trouble dissipating that heat. In hot weather, cool is good. Padded bar tape may allow you to do without mitts, a good helmet may allow more air flow to the head for better cooling, and thin socks will help keep your feet cool - those three areas are good places to dissipate heat from. A top that is thin and slightly baggy will help. Light colors - preferably white - work best.