First, you must find in the literature which genes associates which this cancer and read meticulously articles articulates this issue and
second you will decide which category you are going to focus, like that A) overexpressed genes B) downregulated genes or C) both of them; in a case that you guess there are genes which overlooked, you will design (New possible gene) primer and mRNA extraction until following all the process.
So, thee are easier method that you will focus on the reported genes and your treatment consequent will be published later.
choosing one gene means that you make a strong hypothesis... and get lucky to get some results. why not decide for arrays, which nowadays are cheaper (around 250-300 dollars by samples)?
First you need to know the type of breast cancer on the basis of gross appearance and some diagnostic tests.After that you will go for stage of cancer and then go for literature search specifically. If you still need help, please let me know.