Decide upon a tempo for your beat. Tempo is the speed of the beat and is measured in beats per minute (bpm). 120 bpm is quite up-tempo, 100 bpm is relatively laid back and anything below 80 bpm is quite slow and relaxed.
Choose what timing you want the beat to be in. Most songs use the standard 4/4 timing, which means there are four beats in each bar. In this timing the kick drum is usually on the first and third beat and the snare drum is on the second and fourth beat.
Decide which instruments will make up your beat. Standard instrumentation would involve a kick drum, snare drum and a hi-hat cymbals. However you might want to also include crash or ride cymbals, rim shots, woodblock, whistles, triangle, claps or ethnic drums, such as the tabla.
Add effects to your beats if you want to emphasize certain elements of it. For example, adding reverb to the kick or snare drum will add depth to the song, while delay will cause the drum hits to repeat like an echo for a nice effect.