Bicycle Mechanics - How do I get a cassette off?

While preparing to work on my wheel hubs (see companion thread http://www.bikeforums.net/showthread.php?p=7190436#post7190436 for more), I figure it would be best to repair my sprocket cassette at the same time. ...but I don't see how it's attached. What holds it on and how do I get it off? Also, the wheel bearing under the cassette may be damaged, as I have lost one of the balls and have had three more fall out. Would my local hobby shop probably have bearings as well?

Given the brand of bike (Next) I highly doubt that it has a cassette which means that it will be using a freewheel. All of the department store bikes I've worked on have used the same type of freewheel and use this style of freewheel tool: http://www.parktool.com/products/detail.asp?cat=4&item=FR%2D1

You can pick one of them up at your local bike shop along with the ball bearings you need (just bring the old ones in for size comparison). A hobby shop may have ball bearings too but bearings differ highly in grade which reflects how round they are. You want your bearings round and consistent which means buying decent bearings. Grade 25 (round to 25 millionths of an inch) seems to be the bicycle standard. If your hobby shop has grade 25 bearings, there's no reason not to use them.

FYI, ball bearings won't just fall out of your wheel hub unless you somehow completely lost a cone (impossible to have happen with the wheel on the bike). Your freewheel has bearings in it which might be the source of the balls you've found. Luckily, freewheels are quite cheap to replace.

OK, it's got a little nut this thing slips into. I think I saw it but thought it was part of the bearing. That would really help me out. Wouldn't my bike shop have this too?

I have no idea what you are talking about. Read these links and get back to me:

http://www.parktool.com/repair/readhowto.asp?id=48
http://www.parktool.com/repair/readhowto.asp?id=105

OK, it's got a little nut this thing slips into. I think I saw it but thought it was part of the bearing. That would really help me out. Wouldn't my bike shop have this too?

lol.

As far as wheel bearings, hard to understand how three have fallen out. Unless you have removed the axle, I can't see how they fell out. As for obtaining replacements, you need to go to a real bike shop. Such bearings come in a lot of different sizes, so take one with you.

Better still, just take the entire wheel to the bike shop of your choice and have them repair it.

Pictures would really help..