Think about things like safety (hi-viz and helmet if that's what you want, a good set of lights, and maybe freshen up on your road craft / highway code), breakdowns (carry a toolkit, spare tubes, pump and so on), and comfort (cycling gear and creams can work wonders, especially as certain sensitive areas grow accustomed to time spent on two wheels). Remember: No pants with cycling shorts! Cotton absorbs moisture, which then rubs in all the wrong places. In no time at all you can find yourself in a whole world of pain.
Ease yourself into it: I started with two miles every other night for two weeks, then started to increase the distance. It's amazing how quickly your body adjusts. Soon you'll be doing ten miles, and you'll also start to notice how you're doing more in the higher gears. And once you're comfortable at ten miles, you'll probably find twenty and even thirty miles surprisingly doable. And when you get to that stage, see if you can tag along with a more experienced rider, because you'll learn a whole heap more.
In the meantime, you might want to practice removing both wheels and replacing an inner tube in the comfort of your own home. You'll probably find it's not as hard as you think, and you'll be better prepared for when you need to do so on the road.
Above all: enjoy it and have fun!