When starting a fitness regimen, some common mistakes to avoid include setting unrealistic goals and neglecting proper warm-up and cool-down routines. Newcomers often push themselves too hard, leading to burnout or injuries. It's also a mistake to overlook the importance of nutrition; fueling the body appropriately is crucial for recovery and energy levels. Skipping rest days can hinder progress, as recovery is necessary for muscle growth. Lastly, failing to track progress can result in a lack of motivation and difficulty in recognizing improvements over time.