Safety first, Chester
Judging by your attendance on this web page, you are either a capable handyman or an overzealous, new homeowner about to be made painfully aware of your own hubris. For the sake of the latter, here are some tips to make it a bit less painful.
1. Mind the weather Wait for a sure spell of mild, dry weather. Once it hits, start swinging hammers. Rain will obviously create more danger, but it can also damage your shingles, underlayment, and roofing tools.
2. Always cold? If you must install roofing in the cold, stay off icy or frosted roofs at all costs. Aside from being slick, shingles are more likely to crack and fall out. When removing snow, take care not to scrape of the shingles' protective granules. Since the temperature won't activate normal adhesive, attach the shingles with multi-purpose plastic cement.
3. Power lines Keep your body and tools a safe distance from power lines. Given the extreme consequences of proximity, 'safe distance' will be defined by your common sense. Contact the energy company to have the wires insulated or de-energized.
4. Roofing style Be it slate, steel, rubber, or shingle roofing; you want to be comfortable up there. Wear clothes that don't restrict mobility and won't catch on roofing supplies. Good rubber soled shoes and a brimmed hat are important. Goggles should be worn if insulation is involved.
5. Ladder basics Lay out your ladder on a flat surface and check to see that it is plane and free of defects. Both the ladder and you should be wearing rubber shoes. Geometry tells us, when placing the ladder against the roof, the distance between the base and the wall should be 1/4 the wall's height. See that the ladder surpasses the eaves by about a yard and start climbing - using both hands.
6. While up there Keep the roof clear of debris; this means all unnecessary roofing tools, power cords, and materials. If the roof is steep, get yourself a harness and some anchors - or better yet, a roofer.
More information on roofing