Hi
u know the well known compound H2SO4 , therefore h is +1 hence 2 h makes + 2 , since there is no charge on this compound , therefore so4 has valency of -2 , to add up and make the charge zero.
Memorizing the valences of elements in compounds is an important start. You must also memorize the charges on the compound ions.
The periodic table is organized into families of elements that generally react in a similar fashion so that elements in a vertical column will generally have the same valence. Make a blank copy of the periodic table and enter the symbols of the elements into the table along with their valance(s). A pattern should arise in the table.
As for how to know the valences or oxidation numbers of the elements and ions, the bottom answer is that you just have to memorize them. Once you know the numbers, it's possible to reinforce your memory and come to some understanding of the numbers by looking at a Periodic Table. But really, there's no substitute for doing the work.