I’m another person who disagrees with Cisco - cotton may work great for him but my experience is that for any kind of activity where you’re going to work up a sweat, cotton is the last thing you’d want to wear aside from maybe burlap. It soaks up sweat, does not dry quickly, sticks to skin (leading to nasty chafing and blisters) and will generally make you miserable which will make you want to ditch the whole activity.
This doesn’t mean that you have to spend hundreds of dollars for comfy riding clothes. For $40-$60 you can get a nice pair of bike shorts that will be very comfortable, will wick moisture away (and thus help keep you warm and dry which is critical) and will last a long time (I still have my first pair of bike shorts, Specialized brand, that I bought 12-13 years ago and they’re in perfectly good shape). If you are uncomfortable with the tight spandex look you can either throw a cheap pair of regular shorts on top or get double-layer bike shorts (often aimed at the mountain biking crowd, they’re basically baggy shorts with a built-in bike short liner).
I get a lot of biking stuff at REI, you can check out their selection online and they carry extended sizes, I just took a quick look at men’s cycling shorts and saw things in 2XL, 3XL and 4XL, like this:
As far as a top goes, Target carries plenty of inexpensive polyester workout shirts, both long and short sleeve. I have a number of them and they do the job (feel comfy, keep me warm and dry). $10-$15.
And as far as the bike itself goes, what everyone else said is spot-on; buy at an actual bike shop, they’ll be able to get you onto a well-built bike that fits you, is adjusted to your body and they can make sure you’ve got an appropriate saddle. Again, you don’t have to spend thousands of dollars; for a few hundred bucks you’ll have something good which will last a nice long time. A lot of local shops also give you free maintenance for a year (or more) which is very nice.
All that said it will take some miles to get used to it - just like starting any other exercise routine, you may feel some aches and pains initially so begin gradually and have fun!
Oh, one other thing - if you’re still getting really sweaty/nasty/chafing, try a little Gold Bond powder or Body Glide (it’s a friction-reducer used to prevent rubbing, REI and running stores carry it).