Using my old HDD in a new build. How to clear it?

you format it.

Like when you boot up? In the BIOS? I am pretty green at this...

You can download and use this:

http://www.dban.org/

What Pat said. Just format it once you get the OS installed. Once in Windows, the SSD will be the C: drive, and the hard drive will be D: or something. Just Right Click, and select Format.

With SSD's you don't do a full format - it can wear down your drive. What you do is when you insert your Windows disc and the install menus start up, it should display all the drives and when it does just do a quick format - it'll wipe the drive for you.

And when you format the HDD - once Windows is right click on your computer icon, click "manage" then choose "disc management" and it should display all the discs you have available. Right click the disc you want and then select format, pick a drive letter and you're done.

If you're going to format it in windows like everyone says, make sure you set the boot order to 1) SSD and 2) HDD. Otherwise the PC will just keep loading your old OS install on the HDD during startup. Maybe this is obvious to some but you said you're pretty green  

Yes yes. I ran into this problem when installing my Intel 320, I had to keep setting and resetting the boot order in the BIOS before I finally figured out which order is which.