Muscle groups need attention that is specific to their function and location in the body. Simply lifting weights without proper form will not isolate the muscle you are working on and growth will be minimal at best. Think about what the muscle does, how it functions and keep that in focus when training. Proper form that isolates a muscle should ALWAYS take precedence over the amount of weight you lift.
"Stress" the muscle. Muscles basically grow when they are "pushed" beyond their daily routine and function. They need to be pumped up (isolation is key here) and brought to a reasonable level of fatigue. The muscle has to work until it is tired inorder to grow.
The next phase should make logical sense. Once you have isolated and stressed the muscle you are working on it will need a period of rest. During this resting phase (anywhere from a day to several days) the muscle grows by various biochemical processes that help build it up. Working the same muscle group every day or too often will not add to growth. Rest is critical.
Feed your entire body. Many bodybuilding experts recommend 1 to 2 grams of lean protein a day for each pound of body weight. Honestly, this is often hard to accomplish. You will not believe how much GOOD PROTEIN you have to eat. For example, if you weigh 150 lbs., then ideally you should consume 150 to 300 grams of protein a day!
Building muscle is a great way to prevent many diseases (hypertension, bone fractures and osteoporosis to cite just a few) and stay healthy. You do not have to become a "muscle head" to benefit from weightlifting.
Muscle growth takes time. However, if you follow these VERY basic rules you will see progress in short order. There is nothing like seeing your body change to add to your motivation and commitment!