web site has detailed procedures for all sorts of maintenance and repairs on bikes, so definitely check that out. They also list the tools needed for each repair. I would *strongly* suggest buying/borrowing a good book on mountain bike maintenance and repair, such as Zinn's: http://www.amazon.com/Zinn-Art-Mount...tt_at_ep_dpi_2
If your cogs and cassette (and chain) are worn out, there's a very good chance you also need new cables and (especially) cable housing for both your brakes and your shifters.
Be sure to get one with a reusable easy-open "master link". If you have a "7 speed" bike (rear cassette has 7 gears), a standard "8 speed chain" will be fine, such as the SRAM PC-830. Stay away from Shimano chains, as they do not come with master links.
Tools needed: chain tool to shorten chain to correct size, and to break old chain if it doesn't have a master link.
First, verify that you have a "freehub" type rear wheel, not an older "freewheel" type wheel. Next, determine the number of gears (7 in your case). Count the number of teeth in the smallest gear (probably 11, 12 or 13) and in the biggest gear (probably 28, 30 or 32) -- this determines your cassette "size", e.g. "7 speed 11-28t" means 7 gears with 11 teeth on the smallest and 28 teeth on the largest gear. Again, I'd buy an SRAM replacement on a value-for-money basis. If your wheel is designed for 8/9 speed cassettes, there should be a spacer installed inboard of the cassette -- just reuse it for the new cassette. Be sure to tighten cassette lockring to the specified torque.
Tools needed: grease, chain whip, cassette lockring tool, torque wrench, socket or other means to turn tool with torque wrench.
You can replace either just the 3 geared cogs (if they are not riveted on) or the whole crankset -- you will have to figure out which is cheaper and/or better. For each cog, you will need to know the bolt circle diameter (BCD), how many mounting bolt holes, and how many gear teeth. It's important to tighten the crank bolts to the correct (high) torque value. If replacing the whole crankset, you will either have to get one that matches the spindle length of your current bottom bracket (BB), e.g. 113mm, 118mm, etc, or buy and install a BB suitable for the new crankset.
Tools needed: grease, metric hex key sockets, socket wrench, socket/hex key to match crank bolts, torque wrench, crank puller tool, wrench/socket to turn crank puller. If replacing BB: BB tool, socket to turn BB tool and to attach torque wrench to BB tool.
A great resource, especially to look up unfamiliar bicycle terms, is www.sheldonbrown.com