How to make my external screen the main screen?

Hello all,

Before OS X Yosemite, I was able to do the following:

- Start my macbook pro with the lid close. When logged in I could open up the macbook and it wouldn't open up a second screen.

Now i've tried doing this with OS X Yosemite, but every single time I open the lid of my macbook pro it opens up another "screen", meaning it's mirroring all my stuff. I just want it to open up so the heat can go out, but not mirror from my external display.

Anyone have any ideas?