Cracked my first beer

I couldn't wait any longer. I had 2.5 gal of West Coast Pale Ale (yes, Mr. Beer). I bottled all 8 liters of it and had some left over, so I grabbed a Dr. Pepper bottle, rinsed it best I could, (didn't sanitize) teaspoon and a half of sugar, and put it with the rest. It was hard as a rock after 2 days, and I was able to let it sit for 11 days. This morning, (I work 3rd shift) after 2 days in the fridge, I opened it. I would have to say over carbed, had a lot of head, but tasted flat and a little watery. Don't know how else to describe it. Going into the fermenter tasted good, a little sweet, but good flavor.
Could it be that I didn't sanitize the bottle? I was in a hurry, and all the rest were cleaned and sanitized. Or does it just need more time to carbinate? Hoping the rest will turn out better. I have since bought a fermenting bucket and carboy, plus all the other goodies (hydrometer, autosiphon, ...) and am brewing a 5 gal batch of Weizenbier from Brewers Best. Just trying to learn what I am doing wrong.

I'm pretty sure that for a beer at this stage both flatness and a large head are signs that it hasn't had enough time to fully carb.

That's what I'm hoping for. I like the idea of kegging, but don't know if it will stay fresh while I drink it. That and the boss says I've blown my budget already this year for my toys. Also, I want to make sure I can make good beer before I invest anymore.