You have to start somewhere.
A good way to start a fitness routine it to remember the acronym FITT. It stands for Frequency, Intensity, Time, and Technique.
Frequency:
Start by committing yourself to work out 3 times per week, even if it's only for a short length of time.
Intensity:
Each time you work out, increase the intensity a bit.
Time:
When it is difficult or impossible to increase your exercise intensity, increase the time (or number of reps).
Technique:
Use these three variables (frequency, time, intensity) for each major technique that you include in your workout program (flexibility, strength, cardio, skill training, etc.)
Good Luck. Check the link for a bit more infor.