Congrats! The first ride is always the hardest. I also recommend panniers. These are what I use
http://www.amazon.com/Axiom-Seymour-.../dp/B009P3KQNO
Yes, amazon says they are a single bag, but there are two of them. They are big enough to hold a change of clothes (I've taken 2 pairs before as well), a pair of size 13 shoes, a moderately sized packed lunch, tools/spare parts and then some. They have endured some downpours with no leakage.
I tried a backpack on my first commute as well, and I hated it with a passion.
These bags will fit pretty much any rack, the clips that hold them are adjustable and easily replaceable, getting them on/off the bike is super easy, I really have no bad thing to say about them.