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 may be worth considering installing it yourself.
You may be tempted to take on this project in order to save money, but DIY roofing is unlikely to save you anything. In fact, it could 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. This is why it's important to consider the risks before attempting any DIY roofing projects. It's best to leave this type of work to the professionals who have the experience and knowledge necessary to do the job correctly.