Work from the top to the bottom. By removing the roof first, you can be confident that you haven't already done something to the walls that keep it from supporting your weight.
Check the ceiling on the inside of the shed. If you can see the roofing nails protruding down into the shed, pound them up with a nail. This will make the job on top easier.
Use your ladder to get up on the roof, being careful to confirm that the roof will support your weight.
Tear off the roof shingles. If your shed has a peaked roof, start from the peak. If the roof is flat, start with the ones on top.
Pull off the roofing paper.
Go back into the shed. Using your sledgehammer, pound upward on one panel of roofing lumber. Hit the lumber near where it's anchored to the roofing beams. If you're worried about scuffing, wrap the hammer in a cloth. Your goal is to push the panel up enough to be able to fit in a crowbar or claw hammer.
Pry the panel off with a claw hammer or crowbar. Repeat for the other roofing panels.
Remove any exterior paneling from the shed.
Remove the windows. This usually requires you to pull out the caulk, unscrew the mountings and carefully lower the window. If you're not experienced with this, get help the first time.
Remove the doors and door frames.
Remove the wall paneling, using a sledgehammer and claw hammer, as in steps 6 and 7.
Disassemble the frame. Start with the topmost part of the roof and remove the lumber one piece at a time. Depending on the construction, this may require a hammer, a sledgehammer, a crescent wrench or a screwdriver. Depending on the size and weight of the lumber, you may need to use jacks or a crane, especially for the roof.
Remove the floor paneling, using a sledgehammer and claw hammer, as in steps 6 and 7.