Please make me understand how to calculate valencies of various ions e.g. so4 ,po4, etc

That's a bit complicated, so I'll tell it as simple as possible. Here we go.

SO4

Let's say that sulphur has a valency of 6 and oxygen has a valency of 2. There are 4 oxygen, so, the total valency is 8. Since the oxygen are with the sulphur, the remaining valency will be 8-6 = 2. So, SO4 has valency 2.

PO4

Phosphorus, valency 5, oxygen, still 2. Total valency for oxygen = 2x4=8. Remaining valency=8-5= 3.

Hope this helped!