Most recent distributions should automatically mount your usb devices. If this does not happen in yours, you should use the mount command:
mount /dev/sdb1 /mnt/usb
Where sdb1 is where your usb stick was detected (this could also be sda1, sda2, etc, so check 'dmesg' to see where the device was registered). /mnt/usb is just some empty fodler where the device will be mounted.
Don't forget to umount before pulling the stick out.