Adding to AlexK,
I'd suggest slow and easy. If you use the New Behaviour Generator, set realistic goals for you to accomplish. You want positive feedback that you are actually going forward with this desire to work out!!!
If you wanted to spend 1 hour running, well, that could be your future goal, but, start out with lets say, 15 to 20 minutes of walking. As you accomplish these little steps you
a) mentally have positive feedback and as you accomplish these, pat yourself on the back, literally. (It makes you smile 
, and it's better than kicking yourself in the butt for not working out 
)
b) Physically you will be growing accustomed to the addition strain you are asking of your body to accomplish your workout routine.
Just remember that YOU must take the individual steps to get to the top of your staircase of obstacles.
