from [1] :

ifconfig ath0 up
iwconfig ath0 essid [YOURWIRELESSGATEWAY]
iwconfig ath0 rate 54M
iwconfig ath0 key 8493023948

dhclient ath0

generating WEP key from passphrase , from [2] : 

#!/usr/bin/perl
# Perl script to convert a passphrase to a WEP key.
# By Aki Mimoto

use strict;
@ARGV or die "Usage: $0 passphrase\n";
print generate_key( @ARGV ), "\n";

sub generate_key {
# --------------------------------------------------
        require Digest::MD5;
        return substr Digest::MD5::md5_hex( substr( shift() x 64, 0, 64 ) ), 0, 26;
}

References
[1] http://www.spencerstirling.com/computergeek/wireless.html

[2] http://www.linuxfoundation.org/en/Net:802.11

[3] http://hostap.epitest.fi/wpa_supplicant/