The "Big 4" compound exercises
If I could only choose four exercises to prescribe to anyone seeking to shape up in a hurry, I'd point to squats, lunges, pull-ups, and push-ups. The reasoning is simple: These exercises can be modified for practically any fitness level, they can be done with or without added resistance, they target all the major muscle groups, and they're reasonably accessible with or without a gym membership. Plus, there are about a million variations you can incorporate to help keep your workouts interesting.
Two quick words of advice, though: First, focus on form. If you fail to perform a compound exercise correctly, you open yourself to the potential for injury. And second, work yourself to fatigue. You can do this by adding weight to the exercise or by performing more repetitions per set. Either way, the last one or two reps you perform should push you to your limits. It's when you live at the edge of your capability that real change takes place.