It is as easy as the next few steps:
Change to root using the su command.
run the command: fdisk -l
to list all the drives attached to your coumputer.
Look for the /dev/sda1 line if it exists.
Mount it with the command mount /dev/sda1 /mnt/USBdrive
Be sure to create the /mnt/USBdrive before you attempt to mount it there, as the command will result in a failure if the directory does not exist.
Or you can try to just mount the drive. Plug in the USB drive and mount with the upper command. You can only mount the drive as root, unless you put this line in your /etc/fstab file:
/dev/sda1 /mnt/USBdrive auto noauto,user 0 0
This should allow users to mount the drive and browse it (read and write).
This is all presumed that you have either a stock kernel or your own tailored kernel with FAT partition support and USB mass storage support. When using the stock kernel (distributed with your distribution, if you don't know how to recompile your kernel, then this is what you are using), this is true by default most of the times.
You should now be a happy USB drive user