trying to have a baby

sine i started to get my period i been irregular meanning i would get in about every other month doctors recommended getting on birth control so i can get a regular period. i did for a while and stopped when one of my pap smears was bad so about a year ago i had a leep procedure done then went back on birth control. but now we want to have a baby and nothing. i'm only 24 and we been trying for about 3 or four months. i tried once before for about 6 months and nothing. what can it be