To gain muscle, you have to force it to grow. You have to give it a reason to grow. If you lift 5lbs every day of your life, your body will make sure you have enough muscle and strength to lift 5lbs. You will not give your body a reason to change. How do you give your body a reason to grow? You lift heavy sh * t !!
Girls, it is complete and utter BS when people say you will get bulky by lifting heavy weight. That's wrong. My go-to example is Jamie Eason. She lifts heavy ass weight and look at her. She is not bulky, but has muscles. So do not listen to anyone that says girls should stick to cardio. Jamie Eason has some awesome workout plans you can follow. She is a good one to follow in this industry.
I'm going to use legs as an example when talking about how I have gained muscle. This applies to all muscle groups though.
When starting out I put the hardest, heaviest exercise first. For legs, I am always starting out on squats. I warm up with a light set that I can easily squat 15 times if I wanted to. Mostly I am just stretching out my legs. Then I work my way up the weights putting 10lbs plates on the bar every set. It should take you about 3-4 sets to work up to what I call your “working set”. This is the set where you go balls out, no holding back, kick-ass killer set . Have I made myself clear on how important that set is? Get a spotter on this last set because you're going to need one. Set a goal number of reps – usually between 6 to 12 reps. Take a bit of a rest before starting this set. Now, with every fiber in your muscles rep out as many as you freaking can. When you think you can not do one more, DO ONE MORE. Get to the point where you can not push one more rep out out. You want to be out of breath, tired, and feeling DEAD. That is where muscle growth happens. Then, before you are done with that exercise, drop the weight about 20% of what it was.
On your last set, do 8 reps of 80% of what you just did, then strip it down to a weight that is fairly light, and rep out another 8-12 reps – or until you just can not go anymore (this is what is called a drop set). It should burn. It should hurt. It should make you feel like you want to cry. You are FORCING your muscle to grow. Do this with every exercise you do that day. I promise you will start seeing results.
This is how muscle grows. Not many people are willing to push it this hard. That is why you see the same people in the gym every day that look average, and only a useful who look truly fit.