There has been lots of good advice as to how to balance the boat side to side.
Heres an easy way to balance it front to back:
Pour about a gallon of water inside the boat. The water should sit between your knees and the foot brace when sitting in the rowing seat. This might vary a little depending on the brand of boat, but for most, this is where the sweet spot is.
If the water ends up under the fishbox or worse, you are running too "stern low", if the water slides up toward the front seaters, you're too bow low.
If you're stern low, you'll have a heck of a time slowing the boat down, and it won't want to slide across stream real well.
If you're bow low, the boat will want to spin on you, rather than track straight.
Hope this trick helps you as much as it helped me.