It is generally not recommended for the homeowner to climb onto their roof, let alone attempt to repair or replace it. If you're not sure what you're doing, it can cause serious damage to your roof and, more importantly, to yourself. Most building codes allow for two layers of asphalt shingles to be placed in a house, so removing everything down to the plywood siding may not be your only option. Hanging new shingles over existing ones is less expensive and faster, since it requires less labor, but it also means that the opportunity to evaluate the coating for signs of water damage is lost.
If your roof is in good condition but has some damage on one side due to a falling tree or you've lost shingles in a windstorm, it's worth considering installing it. You may be considering building roofs yourself because you hope it will save you some money. However, do-it-yourself ceilings are unlikely to save you anything. Instead, it may end up costing you more money than it would have cost you to hire a professional.
A small mistake you make on your roof can end up costing you much more in additional damage in the future.