To the OP .... if you want to lose weight, address your eating habits. Or .... be glad you have stronger legs to better carry around the unwanted weight.
As for bikes .... start cheap and see if you like cycling. Why drop $500 (plus $100 for accessories, at least, usually) only to find out that after 20 minutes you hate the sport (pastime, activity, whatever)?
Look for beginner level cycling clubs in your area, and find out not only if they have beginner rides, but if anyone has anything for sale which might fit you. Or shop Craigslist .... and whatever you do, bring along someone who knows something about bikes. Buying a bike which doesn't fit or doesn't work is wasted cash and time and enthusiasm.
Weight isn't a huge deal for a beginner, IMO, because you have no contrasting experiences. That said, I have a sub-17-lb CF speed machine and a 27-lb utility bike and enjoy riding them equally ... so I might be nuts. But the most important part, after fit, is having all the parts work, with everything which should spin, spinning smoothly, and everything which should not move, not moving. That way you will be able to actually ride the bike.
Yes, a heavier bike will take a marginally greater amount of energy to move---but if you are new to cycling, Everything will be somewhat of a physical challenge. Hills are just as long and steep on a bike weighing a few pounds less .... light bikes don't magically pedal themselves.
Get a bike which Works. Ride it. If you can find a slow group, great. If not, just ride yourself. Use a free bike GPS program (RideWithGPS, Strava, and a couple others you can google) and you can track mileage and speed (don't have it visible while riding, because you want to be paying attention to the ride, not the numbers. The numbers are for when you get home.) That way you can track progress.
After a while, you will know if you like cycling in general, and will be able to ask more informed questions, --- and will still get a bunch of garage answers .... but seriously, your first question was just begging for it. Not particularly well thought out at all. No stress .... lots of us us started there (and sometimes go back there.)
Find a cheapish bike and try riding. Then we will know if there is even anything to talk about.