Dear Ameer, you need to buy a book with most used idioms. Learn the idioms and their usage.
If you don’t know the most used idioms in Urdu language and their usage, you will never be able to use them. If you don’t find a book especially on idioms, then buy a grammar book that has a section of idioms. Most of the grammar books contain a special section on idioms, read them, memorize them and then practice to use them in different sentences. This will help you improve your idioms.
In addition to this, also read Urdu Articles in the newspapers and stories from different magazines. This will also help you in other aspects of creative writing.