Easy. Most Linux distributions nowadays feature a partition editor into the installation. So just download the distribution you want onto a CD/DVD/USB, and boot into it. The installer will guide you through partitioning the drive as well. Of course, you are free to partition it yourself prior to installing, but the installer should provide a way for you to partition your drive. Just make sure that you have enough free space to create an extra partition to avoid losing data. So if you have 1TB of disk space and are using 500GB of it, I would partition no more than 400GB of it for Linux. Heck, I'd just get a 100GB Linux partition because Linux apps are so small. They don't take up as much space as Windows apps.