Familiarize yourself with the basics of time signature. A 4/4 time signature is commonplace for hip-hop songs. This means each bar contains four beats, as noted by the numerator---and each note is a quarter-note, as indicated by the denominator.
Listen to songs from the radio or your personal collection to practice counting bars. Without writing lyrics or rhymes of your own, count the beats in each of the songs' measures until you are comfortable understanding where bars begin and end.
Listen to the instrumental to which you are writing, if applicable, as you may write your bars without musical accompaniment. Determine how many beats are in each bar or measure. Ask the composer or producer for the time signature if you are having trouble gauging where bars begin and end.
Count the beats in each bar as you write. Pay attention to how your lyrics fit within each bar. In many cases, one line of rhyming music fits into a bar. Count beats to determine when each bar ends as you recite your rhyme with the instrumental or other music, if applicable.