The squat is a staple strength-training movement that can work your legs, glutes, hips, abs and lower back all at once. To perform a squat, stand holding dumbbells in either hand or with a barbell across your upper back. Bend at the hips and knees and sit backward as though you were sitting into a chair. When you reach parallel (when your thighs are level with your knees), reverse and stand back up. Start with light weight and work your way up, repeating squats for three to four sets of six to eight reps.
A deadlift is another fundamental lower-body movement that can work your legs, back and abs. A deadlift begins with a loaded barbell placed on the floor. Walking up to the bar, grab it with a shoulder-width grip, then sit backward, keeping your chest and head high and your spine naturally curved. Stand with the bar by attempting to push through the floor with your heels, thrusting your hips forward while bringing your body upright. Repeat deadlifts for two or three sets of five to eight reps.
To bench press, lie on a flat bench with dumbbells or with a loaded barbell. Keeping your elbows tucked in toward the body (as opposed to letting them flare out), slowly lower the bar under control to your lower chest. Pause for a second then press the bar upwards again under full control. Repeat bench presses for three or four sets of eight to 10 reps.
If you have the strength to perform pullups, these should be a staple exercise in your training regime. If not, you will have to make due with lateral pull-downs. For either exercise, use a grip that is slightly wider than shoulder width. Initiate the movement by pulling your shoulder blades together and bringing your chest to the bar (or the bar to your chest, in the case of pull-downs). Reverse the movement under total control, and repeat for two or three sets of eight to 10 reps.
If you performed nothing but the aforementioned exercises while in the gym you would be ahead of 90 percent of the people there. Just remember that slow and steady wins the race -- there is no need to make massive jumps in loading to progress. Adding 5 or 10 pounds to the bar every week will keep your body "guessing" enough to ensure continued progress, while keeping you as safe as possible.