Most people breed their heifers at 15 months or so, and they calve at about 24 months. It all depends, however, on the size of your heifer (and also when you want her to calve -- as in spring or fall, preferably NOT the dead of winter or high HEAT of summer -- this could depend on where you're located). And consider a "calving ease" bull for a heifer having her first calf.
Because you're new to all this, please get a book or two on raising a cow. Heather Smith Thomas has a number of good books available.
Good luck on all of this!