That is not going to be very secure.

This is what I use when I need a username or password for some part of my system

#!/usr/bin/env ruby

a = [ ('0'..'9').to_a, ('a'..'z').to_a, ('A'..'Z').to_a, "_"].flatten

10.times do
        puts (0..30).map{a[rand(a.size)]}.join('')
end

Which gives output like:

0Lqimr_6JWoXvFR_UWA0CZo6J23QFci
mwB8_i5N2LPPcHsLQQBfafUUBMZvxxO
nhjWija1r2a_1BSpxhuGOyC3eXIQwjd
d2Jj1mS6ah_OqmWH0J4wL8lOaugfH6t
jZ7_9IYHa9G_JBqha4hMhKo3PnbnMhc
vHjIM925PbqrW_1rOvNLtktSIqdQZQU
ClxbfZp0dg5oxHstqHgfNJyMnPbQTa7
boODNYczqZoNuFeg_ROQ5fj1BPNg3m4
KlBhifcZy_Sl4mFew2e4PBMQasOuBTL
3RZXBYZfmHxiMx1lfBKMsilmIK5vgzN

Pick one for the username and another for the password. Actually I use
a slightly more complex script (using more symbols) but rest assured
converting ordinary text into 1337 text is old hat and even the
dumbest brute force password cracker will try them because people like
you think that b00k is more secure than book.

Also why did you not try and google for this first?