Most home owners will be safe working on a roof with 5 feet of rise in 12 feet of horizontal run. If the roof is more steeply pitched, you should hire a contractor or someone who has equipment that allows them to do the job safely. You may be able to remove larger clumps of moss with a stiff broom or a scraping tool of some type. Be careful not to destroy shingles when brushing or scraping. Asphalt shingles break easily in cold weather.
Shake roofs become slick when wet.
Removing moss prior to applying a moss killer increases control. High pressure sprayers can be used on most roofs. Start spraying at the peak and work your way down the roof, directing the spray down the roof. Getting the high pressure spray under the shingles may result in torn shingles and water damage. In some communities, the local water pressure may be high enough to do a good job with a garden hose equipped with a small nozzle.