Toilets are amazingly cheap, just head down to home cheapo or whatever gets you excited and start shopping. They get more expensive when you start getting into designer shapes and stuff, but what a great opportunity to rid yourself of that lovely avacado toilet they installed in 1978!
Heck, here are the directions even...
Toilet Replacement 101
Clean the toilet, well. Who wants to handle a stinky toilet?
Turn off the water supply valve, make sure it isn't leaking.
Flush the toilet. Flush it again, make sure all the water drains out of the tank.
Take one of those plastic cups your kids drag home from those restaurant kid's meals, and scoop the rest of the water out of the bowl. (If you didn't clean the toilet already, you might want to do it now...)
With a painter's 5-1, or a putty knife, loosen the caulking around the base of the toilet that shouldn't be there in the first place.
Pop the plastic caps off the sides of the base of the toilet, assuming they are still there.
Using a nut driver or socket, loosen the nuts on each side of the toilet.
With a bucket and towel handy, remove the water supply line from the toilet (not the valve end, the tank end!)
Now do the valve end.
The toilet should now be able to be lifted up. I find this easiest by placing my hands inside the bowl and putting my fingers under the rim, towards the back of the bowl at the toilet's center of gravity. Have a sheet of cardboard handy to set the old toilet on, there is a wax ring and some other icky stuff under the toilet so don't set it down on your white wool carpet!
Check the floor for water damage, rot, etc. Check the waste line to make sure it is still secured to the floor flange. Clean all of the old wax ring off of the floor flange, remove the old flange bolts, make everything spiffy again.
Unpack your new toilet, follow manufacturer's directions for assembly.
Place a NEW wax ring onto the floor flange. (DO NOT SCRIMP ON A NEW WAX RING> BAD DOGGY!!!!)
Install the new toilet in reverse of above directions, making sure you put new flange bolts into the flange before you set the toilet.
Do not caulk around the base of the toilet, or you will have no way to know that it is leaking! Some plumbing codes say you have to, because the water trap for the toilet is above this point and the caulk is supposed to seal out sewer gasses. If your wax ring is applied correctly, this is a non-issue.