You've come to the right place! Here's an article on this site to get you started: http://www.tomshardware.com/reviews/build-your-own-pc,2601.html. This link has some suggested builds to look at: http://www.tomshardware.com/reviews/best-pc-builds,4390.html#p1. Here's a site to help pick out parts/prices: https://pcpartpicker.com/. It also has guides and suggested builds, too. Last, there's lots of YouTube videos to show you how to put it together.
You mentioned Windows 7 compatible. I'm not sure if you mean that you have a copy of Windows 7 you plan to use or you mean just a Windows PC in general. If you're building a new PC, I would highly suggest you buy a copy of Windows 10. This is the latest version of Windows and you won't have...