You need to calculate the molar mass. You do this by using the weights that you get off of a periodic table. You have Al2(PO4)3, which means you have 2 aluminum, 3 phosphorus, and 12 oxygen.
Add up all the weights together to get your molar mass of this compound.
Since you have 250 grams, and molar mass is in grams per mole, you need to divide 250 by the number you just calculated. This will give you the number of moles.