I suggest looking at
www.tomshardware.com. They have many, many reviews of motherboards and you can learn what to look for and what the differences are.
Basically, you have to be able to answer a couple of quesitons:
1.) How much $$ do you have to spend?
2.) What kind of processor are you going to put in?
3.) What features do you want?
Most mobo's (mother boards) vary in $$ based on the brand name and also the features that are built in.
The more things built in, the less you have to buy and the more open slots you have. However, if you want to upgrade a built in feature, then you have to disable it and add in a card - which is sort of wasting your money.
IMHO, I would rather add in sound and video. These are going to always be upgraded in technology every year or so. Networking is kind of standardized, so if you're doing "normal" stuff, it's fine to have it built in. Of course you want the latest tech - so go with AGP 4 (I think is the latest), USB 2, and maybe even 64bit PCI.
IMHO, the more PCI slots the better.
Make sure the MOBO is ATX (it probably will be). This is a sizing standard.
Investigate the bios. Some give you a lot more control over your machine than others - this is especially pertinent if you ever have any intention of Overclocking the CPU.
Factor in what kind of memory the board will take and how much. Some types of memory require two banks - another price consideration. Older memory is of course, cheaper, newer faster is more expensive. Remember to look at what kind of memory it's going to take.
I would assume you're going to be putting in WinXP? Check out M$'s hardware compatability list. Also, once you have some choices, get other opinions. Here's a good link for this:
http://discuss.extremetech.com
HTH!
Have Fun!