Most new toilets come in two boxes: one for the bowl and one for the tank with all the moving parts installed.
The steps here show how to assemble the tank and bowl and how to install the toilet. You may install the bowl first and then attach the tank. If the existing toilet is heavy, you can detach the tank from the bowl before removing it. Keep your back straight and lift with the legs. Arrange for a helper if possible. Follow these same steps if your toilet is leaking from the base and you need to replace the wax ring.
You may install a standard, pressure-, or pump-assisted toilet. The rough-in distance -- the measurement from the drain center and mounting holes to the back wall -- is 12 inches for most toilets. Some are 10 inches. Measure the old toilet and buy a new one with the same dimensions.
About 2 hours
Adjustable wrench, hacksaw, utility knife, putty knife
Making simple plumbing connections, assembling a toilet
Shut off water to the toilet; flush; use a sponge or rag to remove all the water you can from the tank and the bowl
Toilet, wax ring, flange bolts