We've put together some ideas to get you started, but there are lots of other articles online. Do your research before you start, and consider your circumstances. For example, you may be using the deep litter method, or may be cleaning the coop in response to illness in your flock, in which case you may need to modify your cleaning procedures accordingly. With that in mind, here are some basics to think about.
Gather your cleaning supplies: garden fork or shovel; bucket and scrubbing brush; assorted rags for dusting; disinfectant of choice; wheelbarrow or garbage bags. You are going to get very dirty, so wear suitable clothing. Consider a dusk mask, as well, to protect your lungs.
Disconnect your electricity. Thoroughly wash the coop, including the walls, floor, roosts, ceiling, nest boxes and windows. Go behind any crevices that could harbour mites or other critters. Clean under any flooring you may have installed. Rinse the coop, and mop up any puddles.