I'm assuming you're talking about dual-booting Windows, rather than another flavor of Linux, correct?
As frankbell said, you don't really uninstall an OS, you just reformat.
- You'll have to boot from either a LiveCD or a Linux-On-Flash-Drive to make these changes, because you can't resize a partition while it's in use.
- Resize /dev/sda1 so that it takes up half the disk (I've found some instructions here).
- Create a new partition in the free space you just created.
- Install Windows (or whatever you're trying to dual-boot) in that partition.
- The Windows install will clobber your MBR without asking, so you'll have to reinstall Grub to get Debian working again (either that, or use a boot disk whenever you want to run Debian).
Those are the basic steps, feel free to ask if you get stuck on any one step (except for the "Install Windows" step