In order to do anything with your usb device (I assume you mean a USB Flash memory), you must first mount it in your linux filesystem. All the ones I've seen so far were formatted as FAT filesystems, so as root:
mount -t vfat /dev/sda1 /some/existing/directory
The descriptor for the USB stick is /dev/sdX, where X is 'a', 'b', 'c'.... If you already have a SATA or SCSI drive, it will be sdb, or sdc ( depending on the number of previously installed drives (ATA drives are designated as /dev/hdX). The designator for the partiton on the USB stick will almost certainly be '1', as I used in my example '/dev/sda1'. This would only change if the USB stick had multiple partitions, and if you wanted to use the second or higher partiton number. The mountpoint is any existing directory that you wish to mount the device at. A common practice is to create a '/mnt' directory, under which various specific mountpoints are created, such as '/mnt/cdrom', '/mnt/floppy', or '/mnt/usbstick'. You are free to use whatever mountpoint you see fit. Once mounted, the device can be read or written as if it was any other disk media.