Just a afast hello from another beginner! First things first - I came to this forum simply to ask if I was too big to ride (bout 378lbs) - and as you see above, you get a lot of kind help here!! Mostly I would simply say what others have said above, but a few tiny points to add if I may?
Check the tire pressure each time you are going to ride. If you look carefully on the side of the tire, it will tell you the maximum PSI the tire can hold. A good new cheap air gauge at Wally world is 3-4 bucks, and fairly accurate! Having those tires (any tires you use) at the highest allowed pressure is going to be good for the bike and YOU! - The bike = cuz the tires will get less flats, you = cuz the bike will roll easier, which will make it easier to ride. Harder tires do make for a harder ride, but (to me) that is a fair trade.
The only other point if I may? Even if it's for 5 minutes, get on your bike every day. Some days you will have time, the weather will be nice, you will feel great, and you will get lots of exercise, other days you're in a hurry, or feel sore, or something is wrong - still, some time in your day, take at least 5 minutes and get on the bike and ride even just up and down your drive-way a bit.
This will get you to the place most of the folks here get, where they are happy when they ride, and cranky when they don't. I am having a knee replaced soon, have much weight to lose, it's been 30 yrs since I rode, and I am starting - so yup, you can (as you have told us) too!
Remember - you can find reasons every day to NOT get on the bike, the secret is to find reasons every day TO get on the bike!