implementing an encryption algorithmn

[Pouncer]

I'm a bit unsure on how I go about implementing this.
Say I want to encrypt this string:
"Good morning"

with a key: "abc" where:
a = shift 0
b = shift 1
c = shift 2
etc..

I want to shift each letter like this

Good morning (original string)
abca bcabcab (keys)

Gpqd nqroknc (the resulting encrypted string)

As you see, each letter is shifted depeinding on the key letter.
Can someone give me some ideas on how i implement this encryption? I would much appreciate it.