First decide whether you want this drive repartitioned. You can use gparted to repartition and reformat the drive. Or you can use "sudo /sbin/fdisk <device>" to do it from the console. Delete the original partition if you will be repartitioning it. Make sure that the partition isn't mounted first.
After partitioning, you need to create a filesystem on the partitions. I would stay away from reiserfs. I don't know how long it will be around. Ext3 or Ext4 are the defacto filesystems on Linux, and while ext3 isn't as fast as others, may be more robust which is important for an external drive. Ext3 has been around a lot longer than ext4 so it is proven.
On the other hand if you will be using this for large media files, another filesystem may be more suited.
Assuming one partition, ext3, and it's node is /dev/sdb1:
sudo mkfs -t ext3 -m0 /dev/sdb1
The -m0 is an optional option which doesn't reserve some space only for the root user. Since this an external drive, you don't need that spare space.