In reply to sweenyt:
Good for you - with any luck you'll soon find that the thought of NOT cycling in becomes abhorent.
My commute is a bit easier (10 miles with 700ft of climbing, only one way) than yours. My tip is not to throw yourself in at the deep end. Start by doing one day a week at first, then two days a week, then three days a week etc. Build up gradually. The first day won't kill you, but if you jump in and start trying to go everyday, then by day 3 you will hate your bike, hate yourself, and give the whole thing up.
Commuting is very different to riding for pleasure, mainly because that's not what your doing - you're riding with a purpose beyond 'fun'. That's not to say you can't enjoy yourself, but it's also so acknowledge that the commute home on a wet wednesday evening, when you're tired after a long day at work, may be more difficult than you want it to be!
You'll find you start eating twice as much. In my book, this is a good thing!
Get some decent lights. Unless you're going off-road you don't need stadium jobbies, but you need something sturdy and strong.
A backpack is fine, but if your back starts hurting or gets unpleasantly sweaty, you may choose to invest in a rack and a bike-bag/panniers.
If you don't have a shower at work, then a flannel, a towel, and a bathroom with a private sink is perfectly adequate.
Keep a permanent stock of at least two emergency energy bars in your bag (in case you run out of juice on the aforementioned wet wednesday evening - trust me, it's a horrible experience)
I don't know what bike you have, but get some cheap, hardwearing road tyres that resist punctures (i.e. neither mtb nobblies, nor race-light slicks). Keep them pumped up hard.
Oh, and keep at it, it gets easier, and you get to live with a "I'm permanently really fit" smugness...