1. Check which drive it is (/dev/sda3, /dev/sdb1, etc) I always do it with 'fdisk -l'
2. Create a directory where you want to mount it, eg. /home/user/disk
3. As root you can mount it
mount /dev/sda3 /home/user/disk
There might be some specific mount options depending on the filesystem of the drive
What file system are we talking about? Is it a windows drive?