Welcome to cycling!
Don't worry, most normal bikes will handle someone of your size without any danger.
Your best option is to buy a cheap used mountain bike or hybrid (Mountain bike/street bike) for as low a price as you can get. Your local bike shop may have used bikes or take a look in the local classifieds. Don't worry too much about components or brands at this stage.
The bike should have 21 to 24 gears; 3 chainwheels in front and 7 or 8 cogs on the rear wheel. This is adequate to get started.
Make sure the bike is the correct size, i.e. you can stand over the crossbar with one inch of clearance with your feet flat on the floor. Check that it steers straight, and that the brakes work before you buy.
Once you start, it's important that you wear a proper cycling shorts so that your butt doesn't get too sore as this will put you off biking. You can also buy padded inserts for shorts if you can't find suitable cycling shorts.
Once you start to get fit, you'll outgrow your first bike and gain knowledge about what type of bike is most suitable for you long-term.
Don't forget to take it easy: fat only gets burned at 65% or less of max heart rate, this is less than the rate at which it becomes difficult to carry on a conversation. If you cycle on your own, sing a song as you pedal, and if you become breathless slow down until you can carry on. It's also a handy way to practice your party piece
Best of luck,