CrossFit is a strength and conditioning program based on broadly varied, functional movements performed at high intensity that is universally scalable for all levels of fitness.
Fitness in 100 words:
Practice and train major lifts: Deadlift, clean, squat, presses, C&J, and snatch.
Similarly, master the basics of gymnastics: pull-ups, dips, rope climb, push-ups, sit-ups, presses to handstand, pirouettes, flips, splits, and holds.
Bike, run, swim, row, etc, hard and fast.
Five or six days per week mix these elements in as many combinations and patterns as creativity will allow. Routine is the enemy. Keep workouts short and intense.
Regularly learn and play new sports.
Eat meat and vegetables, nuts and seeds, some fruit, little starch and no sugar. Keep intake to levels that will support exercise but not body fat.