If you are new to TrainingPeaks, or have not been tracking your Training Stress Score you can estimate your Fitness (CTL) starting value.
The appropriate starting value can be estimated by realizing that most people train at an intensity resulting in 50-75 TSS per hour (i.e., average weekly IF is usually between ~0.70 and ~0.85). Those who train more, mostly or entirely outdoors, and/or in a less structured fashion would likely fall towards the lower end of this range, whereas those who train less, frequently indoors, and/or in a more structured fashion would tend to fall towards the upper end of this range.
Unless there is a specific reason to do otherwise, the same value should be assigned to both Fitness (CTL) and Fatigue (ATL).
The calculated values for Fitness (CTL), Fatigue (ATL), and Form (TSB) should be interpreted cautiously following such a “seeding” until sufficient data are available.
If you are using the ATP and you have 4-6 weeks of data recorded then the ATP will automatically use the correct starting CTL for your ATP starting date and you won't need to enter an estimated value.