This reminds me of the question, "When is the best time to plant a tree?"
The Answer: Twenty years ago -- and the second best time, is now.
In planning, it's sometimes difficult to decide where to start. I advise starting with something easy to accomplish, that you have time for, that you'll keep up with. The Daily Dozen was designed for this reason.
As far as when? You can start today! Right this minute, if you want to. Do it!
Here is a link Daily Dozen
Fit By Nature covers this in the 'Getting Started', section. Do it! Whatever your fitness level is today, you'll feel great that you did something good for yourself.