An anterior placenta is on the front of your womb. Posterior placentas are on the back of your womb.
Having an anterior placenta does NOT mean you are 'doomed' to a posterior birth!
That is but a theory...people are always trying to figure out why some babies like to present posterior (with baby's back toward your back, instead baby's back toward your front). There are many possible reasons/theories on why posterior presentations happen--but last I knew, no one really knows for sure.
Yes, in SOME cases, placental placement can affect baby's position--but I don't believe it is possible to predict this.
Besides, posterior positioning, while 'famous' for creating harder labors, is not necessarily going to cause a harder labor or be any problem at all.