Hello and welcome to the forum.
If you're overweight you need to sort out a diet. Find one that appeals and stick to it. You won't loose weight by cycling alone.
As above start the cycling with short trips that you can enjoy and build it up and use it to go shopping as well. Your legs will feel it at first but you'll get used to it.
Make sure you have a comfortable position on the bike. As a guide to saddle height your leg should be more or less straight when seated with the pedal at six o'clock.
Anything else you need to know just ask.