I have an Amana, fridge/freezer (top/bottom) combo and the freezer door is not staying shut. After shutting it, it opens slightly and is frosting all inside.

I wasn't sure what is resopnsible for keeping the door shut, are their magnets in the door, suction... any help to keep my door shutting would be greatly appreciated.

Generally the magnetic seal around the door is what keeps it closed. If it is not sealing properly, it is probably because the door is warped.
Remove the door at the hinges. Lay flat on floor (exterior side down) with a drop cloth or blanket to prevent scratching. Remove any handles or other things that prevent it from laying flat on the floor.
Lift the edge of the seal and loosen all the screws around the edge that hold the inner liner in place. This will allow the door to return to flat. The inner liner is what gives the outer shell its shape. After loosening (you don't have to remove, just loosen) all the screws go back and tighten them. The door should now be flat and should close and seal properly.