One of the most important weight training exercises you should do, especially if you're a beginner, is the squat. Squats not only work the quadriceps muscles, but also the lower back, glutes, and hamstrings. The upper body muscles also get benefits from squatting as the heavy breathing involved if you do high rep squats may actually increased the size of your rib cage (which will make your pectoral muscles and the rest of your upper body appear larger). You'll also get a testosterone boost from squatting which will help you gain more overall muscle size and strength.
While performing squats during your weight training workouts you'll need to use the right foot placement. You can either place the feet at about shoulder width or wider than shoulder width.
Placing your feet wider than shoulder width will hit the large quadricep muscles on the outsides of the upper thighs and will heavily involve the glutes, hips, and hamstrings in the movement. This is the way most powerlifters squat and you'll be able to squat heavier weights this way, but you may get more muscle growth in the glutes (which may make your butt too big) than you want.
If you place your feet at about shoulder width while squatting you'll hit the quadricep muscles at the front of the upper thighs as much as the outer quadriceps muscles. If you squat this way you won't be able to use as much weight, but you also aren't as likely to use the glutes as much. Use the foot placement that you feel more comfortable with while performing squats, based on whether you're trying for more powerlifting strength or trying to make your body have an improved shape.
To properly perform squats you'll want to go down to where your upper thighs are parallel to the floor or deeper with an arch instead of a bend in your lower back. It's very important to avoid bending your lower back as much as you can. You'll also want to place the barbell on your upper back and trapezius muscles rather than on the back of your neck while you squat. Most gyms have pads to cushion your upper back muscles when you squat, though squatting without one should make you feel like you have more control of the weight. Ankle flexibilty will partially dictate how deep you can comfortably squat - more ankle flexibility will allow for deeper squatting. You likely won't be able to squat to far beyond parallel if you have long upper thighs as well.
You should always have a weight belt handy while you perform heavy squat workouts to help support your lower back. If you're not squatting too heavy and doing higher reps, you may not want to use a belt as they may make it harder to breath heavy due to tightness and might not feel comfortable. You may want to use knee wraps if you're squatting very heavy, but shouldn't use them if you're doing high rep sets as they may numb your lower legs if you have them wrapped tightly.
If you're not competing in powerlifting competitions and have squatted for many years, you may eventually want to stop doing regular squats in your weight training routine. Years of heavy squatting may give you a big butt and will likely give your lower back and knees a beating no matter how much you support them. If you've been performing squats for a while you should have enough muscle size in your legs to start using squat alternatives like smith machine squats, hack squats, and leg presses as your primary quadricep exercises. All of these exercises will allow you to focus on the quadricep muscles directly without involving other muscles too much.