Tap your toes up and down while sitting on a chair. Practice this for a while, seeing if you can keep a steady pace.
Count to four out loud while tapping. Each time your toes hit the ground is a new number. In 4/4 time, each count is a quarter note. Most rap and hip/hop music is in 4/4 time. This means you can count to it by saying "1 - 2 - 3 - 4, 2 - 2 - 3 - 4, 3 - 2 - 3 - 4, 4 - 2 - 3 - 4." The first number on each group of four numbers represents the measure.
Play a song and listen to the drums, bass or rhythm guitar and practice counting, tapping along with the beat. It also helps to throw in a little head nod.
Sync the lyrics to your toe tapping. This means you should be able to tell when a line of vocals will finish and get ready for the next one. A line of lyrics will most likely finish when you hit the fourth down beat. This is because most songs follow the rule of measures.