The subject of retrosynthesis is so vast and your question is so vague, that the only way to possibly help you would be to write a textbook.
I have you covered.
For basics of retrosynthesis, read chapter 30 pages 771 - 801 in Clayden, or search the appendix at the back for the chapter in whatever edition you find.
Also, why not post a simple retrosynthesis with your attempts and I or others can walk you through the question, but you have to show an attempt first.