You can build a filesystem on that partition with the mkfs command. Man mkfs for more information. You also could directly invoke the particular format utility with, for instance, mkfs.ext3 /dev/sda1.
If you wish to use this partition to extend /usr and /home, then pick the portion of /usr that you want to move, and move it to sda1, then symlink to it from the old location. Similarly with /home.