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To have to learn how to make the electronic configuration of that element
for that you have to know Afbau's principal and then you can get the number of electrons in the last shell and then you can get any elements valency.
thank uu very much .........
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See here now as my brother told me when i was in 6th how to bcme very good in valency is the following steps and with these i am sure you will learn it very well:
1) first make two columns
2) write down all the metals with its valency and variable valency by seeing ur text or whatever means u have for the info 
3) write down all the non-metals ans radicals on the other column
4) learn the criss-cross method to write a formula.( u shld absolutely knw this)
5) not start with the first metal (for example K) and then keep doin the criss-cross method with that metal only till u finish the criss-cross wid all the non-metals with that first metal.
6) and whoalla u will become perfect wid ur valencies