You're getting a lot of good advice, but little in the way of truly practical instruction.
One point to keep in mind (until you learn a bit more) is that the weight of the hammer head is designed to sink the nail, not the force of your blow. I, personally, can repeatedly sink a true, fat 16 penny common with only a tiny tap to set the nail, and ONE hard, serious blow. But you will become quickly frustrated if you attempt this before driving nails all day long, every day for at least six months.
So... what to do?
Start with this exercise: You have been told to drive with your arm, and not your wrist. This is true. But what does it mean?
It means that when you grip the handle of a hammer you should concentrate on "locking" your wrist so that it neither bends forward or back, nor rotates to the left or to the right. It, in effect, becomes a frozen extension of your forearm.
Here's the exercise: Leave all your nails in their box. You will not be using them for a long time yet. Position a small pillow on a workbench so that you can place your elbow comfortably upon it. Put your elbow on the pillow. (The pillow is simply to keep you from rubbing your elbow raw right now... USE IT.) Grip your hammer near the end with your wrist tilted forward like you were holding a fishing pole. Now bend your elbow till your forearm is vertical. Good.
Now, keeping your forearm and wrist stiff and firmly aligned with the 'fishing pole grip' of the hammer (one continuous piece of bone held that way by your muscles) let your forearm, wrist, and hammer simply fall to the workbench surface.... "thump!"
Observe how 'flat' the face of the hammer head contacted the workbench surface. If it was not flat, adjust your grip some to rectify that misalignment. Bend your forearm back up and do it all over again. (Your elbow has never left the pillow.)
A point to watch here is that your wrist does not get weak or sloppy at the moment the hammer head impacts the workbench. This is the biggest cause of missed or bent nails. After the blow, with your hammer head resting on the workbench surface, your whole unit of forearm, wrist and hammer handle should remain rigid and in the same orientation (relative to each other) that they were in when you began your hammer 'drop'.
Remember, during none of this are you adding any effort to the simple force of gravity. You are merely letting the hammer drop. Nothing else. And your elbow is staying right there on that pillow. Don't you dare let it lift up any.
Do about a hundred of these.
What you will learn from this is that the main motion of hammering comes from just your elbow. Notice I said 'motion", NOT... 'force'.
This was lesson one. After you become fully comfortable that each and every blow of your hammer will hit a spot on the bench that your are looking at (a target) firmly and solidly, you can tack in a small 6 penny nail about 1/4" deep, and try using what you've learned to drive that nail down into some wood.
Do NOT try to drive the nail with one blow. Let the weight of the hammer head eventually sink the nail home. It will probably take about four 'drops' of the hammer head.
As you progress, you will naturally and instinctively begin to incorporate some extra muscle into this exercise. This is cool (but only after a few days) as long as you keep in mind that it is the hammer that does the nailing, not your arm or shoulder.
Get good at this, and you can do away with the pillow. This will be a little hard to do at first. Just remember that even without the pillow, your elbow is still supposed to stay positioned in one solid location.... even if it is up in the air.
Stick with this, and you will be surprised at what a fantastic nailer you can eventually become.
Keep practicing this until you are confident that you can maintain total control of that one long, solid unit of forearm, wrist and hammer handle.
