For girls beginners you can start immediately with squats and exercise with a blank fretboard. Correct technique of performing sit-UPS blades flatten, sit down, bending body in pelvic and knee joints, back, forward. Lower back not rounding, keeping a slight natural curve in lower back this can reduce stress on back and transfer it to gluteal muscles and quadriceps. When lifting, knees should not be reduced inside. After rising to exhale.