Ngh, feel bad because I just came back from barely being able to sew, but I guess a fellow n00b can help out on this point? ^^;;
I would definitely recommend sewing classes - they stay with you for years, so even when you want to sew again after five or six years, you'll still remember <i>something</i> and can go off of that. JoAnn's is a good place to start - my aunts went there and recommended it to my mom. Have you thought of just asking around the area as well? I know in my town, they sometimes have community centres or even community colleges where they teach sewing. That could help as well.
And whether you go to a sewing class or not, you should get a book either way, or at least get very well acquainted with google. Sewing Basics by Patricia Moyes was what we had in the closet, so that's what I used when I picked up sewing again. Pretty good in covering all bases, and has clear pictures. I also used google. A. Lot. It's pretty amazing what tutorials you can find on google, from the basic to the esoteric (example in point: I recently sewed in a zipper, but didn't want to buy a zipper foot. I managed to do it without a zipper foot, and it looks pretty good especially for a n00b like me).
Finally, can't give you much on the fabrics you bought. Muslin is used more as a lining and practice than an actual clothing. If you want a very good costume, it's generally frowned upon for good material, but it is very easy to use for beginners. I used muslin for my cape recently because I am new, I didn't want to ruin good cloth, and because it was easy to work with (I had to dye the cape, so if I ruined it, I could go back and get some more without mourning the loss of moula too much. Luckily, it held in the dye very well). Broadcloth is a lightweight fabric - densely woven cloth - and although it does have its place, I've been advised against it in the making of whole costumes (you can quickly google broadcloth cosplay to get an idea of what people think about it). I'm not quite sure what cotton you might have bought - there are tons of different kinds of cottons. But as practice? Those are all good fabrics. They don't slip, they are strong enough to take lots of stitch removing, and are pretty easy to sew on.
The thing about good fabric is that it's expenseeve. Like, really expensive. You'll want to practice lots, probably sew several projects together, to attain the sort of confidence and nerves of steel you'll need to attack good cloth. I was once briefly considering upholstery fabric for my costume. I shied away from it because, yes, it was twice as much per yard as bottomweight cotton. So just a heads up if you want really good cloth - you'll have to pay through your nose for it >.<
Ergh, hope that helped and wasn't just a semirant that went nowhere...