Create the access list :
access-list inbound permit tcp any any eq 2121 access-list 101 permit tcp any interface outside eq 2121
For the port forwarding,
– 192.168.1.x = Adresse destination
– 2121 = port source
– ftp = port destination
static (inside,outside) tcp interface 2121 192.168.1.x ftp netmask 255.255.255.255 0 0