Right. Basically, to carbonate yr beer, you have two options.
1) Given a set temperature and a set volume you add x oz priming sugar. The yeast eat the sugar, and poop the co2, and over time, the co2 absorbs back into the beer.
2) Given a set temperature and a desired carbonation level, you apply forced CO2 (from a tank) to your keg of beer over time. The CO2 absorbs into the beer over the course of 1-2 weeks, and your beer is carb'ed.
Brewing software helps dramatically with both of these - I use BeerSmith, and would be incompetent without it - but there are DIY calculators (Excel spreadsheets and web-based calcs and the like) that are avail. for free to help, too.
That's it in a nutshell - I would go into more detail, but I've been at the bars this eve, and so I should keep my post short so I don't start saying wrong info.
Oh. And the "Major" online HBS'es are... Northern Brewer... Austin Homebrew... Midwest Homebrewing Supplies ... MoreBeer (also called B3, from when they were "Beer, Beer, and More Beer"), and Williams Brewing. KegKits is also a great link to have handy.
Generally, whenever I pick up new gear, I check the price on all of the above before pulling "the trigger", unless I need other stuff that I know is cheapest from a certain source.
Cheers, and please feel free to ask whatever else may be on your mind, I'll try to keep an eye out for your ?'s! Welcome to the hobby, hope you love it!