You: How far will you be traveling each way to or from work?
If you are a true "out-of-shape" novice as I was, plan on a speed of no more than 12 to 15mph. So three miles will take about 12 minutes, 5 miles will take about 20 minutes, and 10 miles about 40 minutes (don't worry, it will go much faster after 8 weeks). Depending on your physical condition and the distance to work, you want to start off easy, say three times a week. Santa Barbara Traffic Solutions has a yearly month-long bike to work challange. Most participants begin by riding 5 days a week. Most novices are able to do this, but it is a challange! I actually recommend this "cold turkey" approach, wake up one day and never go back! At any rate, within 4 weeks you should be riding every day.
If there is any question about your physical condition, get a checkup ask your doctor if there are any health issues that would limit your riding.
This is where I think most people, with the best of intentions, loose it. I had several false starts over twenty years. These were largely due to my choice of a bicycle. There are three things that you want from your bike:
1) it is right for the job - fits you properly and is appropriate for commuting,
2) it is a quality bike that will stand up to you, the road, and time, and
3) you really like it
You are almost ready... Since you are a commuter, you will likely need to take "stuff" to and from work. For me, it's at least a change of clothes and whatever homework I brought home. This will require a backpack or basket.
The other pages of this site will help you get the details you need to get better prepared for your new life. What about cold weather, darkness, rain (we don't have that white stuff). This is a change in lifestyle, make no mistake. Once you get going, you will find it hard to stop.