Welcome aboard! If you give yourself half a chance, you will love cycling. It's a great way for someone who's overweight to get back into shape because it's not as hard on the joints as running and a whole lot more fun than walking.
Here are some clothing suggestions.
1. Whatever you do, don't ride in those sweats. They will quickly turn clammy with perspiration and make you feel miserable, especially if it's already cold outside. And then you'll hate cycling before you even know what it's like.
2. If you don't want to ride in tights but it's not warm enough for shorts yet, you may be able to find some workout pants that are made from a blend of synthetic fibers. Mine are called "Not Quite Tights" and I got them from Travelsmith (online), but your local sporting goods stores may have something similar.
3. Good cool weather choices for tops are, as someone else mentioned, a wicking undergarment--something like Coolmax, which is a brand name, but there are others that work just as well. If you can't find things without any cotton content, at least look for low cotton content, blended with some synthetic fiber. Fleece makes a good outer garment--it will keep you warm even if it gets wet. An old wool sweater would work too--it's the traditional alternative to fleece.
4. For weather that's borderline--not too cold, not too hot--check out some of the cycling knickers that a lot of companies are selling now. I have two pairs of Terry knickers that are my favorite cycling "bottoms." They're just as tight as traditional cycling shorts, but the extra length makes them feel less revealing. They're also very flattering, even on my not-so-svelte, middle-aged legs. They come in a wide range of sizes, including plus sizes. Check out http://www.terrybicycles.com/ and www.teamestrogen.com
5. Another comfort consideration will be your saddle. Check out other threads on this site for useful saddle information. Especially, don't be afraid to ask your bike shop whether they'll let you exchange a saddle that doesn't work for you, until you find the right one.
6. Have fun!
--J**
"Do I contradict myself? Very well, then, I contradict myself." --Walt Whitman