Welcome to the forums.
You are asking a loaded question. Each situation is going to be unique. You also need to have a good base understanding of all the relevant subjects. If you search this forum, you will find a ton of links to various book, online articles, challenges, etc that will get you started.
I like this pen testing framework:
http://www.vulnerabilityassessment.co.uk/Penetration%20Test.htmlIt offers a very good guideline and even links you to some of the tools that can be used to audit the relevant devices you will find.