how do i format a harddisk (slave-drive) just installed?

By system, do you mean the operating system, or the computer?

Assuming you have the operating system installed already, and you recently added a hard drive, here is what you can do. Install the Gnome Partition Editor; you can do this in Applications > Add/Remove Applications. Then you can use it in System > Administration > Partition Editor (you'll need your password, since this modifies the core of your system). This is GParted.

There will be a drop down menu in the top left, second row. Select the appropriate drive from there; it gives you a hint with the size, but you might need to test each to see which of the drives is empty. After selecting the new drive, you'll see an item called "Unused" (or something similar--I can't remember well) in the table. Right click on it and select Format to > ext3 (or any other format you prefer). I think you'll also have the opportunity to give it a mount point, which is the directory that will correspond to the partition. That should be it.

You can also resize that first partition and create another next to it, to mount it somewhere else. Or other combinations. Click on Apply when you're finished; you'll have to wait for quite a while depending on the size of your hard drive being modified.

Good luck! :)