How to set up my iMessage account on my MacBook Pro?
This support page from Apple should help you out greatly:

http://support.apple.com/en-us/HT201349

This page should help you set it up on OS X (it's using OS X Mavericks to explain the instructions, but it's the same steps if you have OS X Yosemite), and will give you troubleshooting tips for if your phone number is missing on OS X.

Hope this helps.