to mount it you would need to add an entry in your fstab file (/etc/fstab)
something like this, although it is possible it will vary from this. you can man fstab for more information
/dev/xvdf1 /var/postgres ext4 defaults 0 0
If you do a df -T you will get all the information you need
/dev/xvdf1 ext4 51605436 26338144 22645888 54% /var/postgres
so you know that the device is xvdf1 (it may be sdf1 or something like this) the second entry is where you want it mounted, the third entry is the filesystem type (which you can see as the second entry in the df -T command) and if you don't understand fstab, it's probably best to leave it as defaults and 0 0
Hope this helps.