mkfs after fdisk?
 

I installed a new harddrive.
created a new primary partition for the harddrive using fdisk.
mount the new harddrive using "mount /dev/hdb1 /mnt/hdb1"

after that I can use it already?
don't I need to run "mkfs??"

or fdisk automatically assignes a file system "ext2" default

Yes, you need to make a filesystem for your partition before you can use it with mkfs.