Hi jorasmi,
You simply have to make a filesystem on the partition you want to use. Let's say, the partition is adressed via /dev/hda6 and you want to "format" it with reiserfs, you just have to be logged in as root and execute
Code:
mkfs.reiserfs /dev/hda6
For other filesystems, there are similar commands, for example mkfs.ext2 or mkfs.ext3. If you're unsure which filesystem to use, I recommend ReiserFS. It's fast and journaled :-)
Greetings,
Seppel