Ok, I would suggest going to a boxing gym etc
1. You need to fix your stance, im going to assume your a righty.
a. When you face the bag your left foot should be forward and should aim past the bag going right. Your right foot shouldnt be at a 90degree angle away from the bag but more angled away than your left. Imagine a straight line going from your left foots big toe to your right foots heel. Your feet should be about your shoulder length apart.
b. Keep your hands up. Your left hand should be at your cheek and your right close to your chin.
c. Bend your knees a little bit, you want to be able to step forward with your jab or go side to side to simply hop away from your opponent.
d. Keep your chin down and tucked, if you have to take a punch your best spots to get hit above the neck are the forehead and the top of your head as they are the most resilient and will cause you the least damage.
2. Combinations
a. Practice your jab - when you jab its called a 1 - i like to step forward as I jab. It should be crisp and fast as fast as it goes out is how fast it should come in.
b. The right cross called a 2 - your essentially throwing a right punch but your twisting into it from the hips. When you throw this your left hand should never leave your cheek and by twisting into this you are protected from hooks but you have to watch for straights and uppercuts, this is best used in combination after causing them to cover up with your jab.
c. The 1, 2 - step forward Ljab (bring it right back to your cheek) with a right cross. Step back to avoid the counter.
d. The 1, 2, 3 - Ljab (bring it back to your face) + right cross (bring it back to your face) + Left hook. This combination though simple is amazing for starters, it works best when you use the momentum from your cross and explode into a left hook.
3. Head movement, bobbing and weaving - You need to keep your head moving but change it up if you move the same way everytime you'll end up leaning into the end of your fight.
Get in front of a mirror and practice ducking, movement and try to find how you would anticipate your own movements and then change.
Im going to short you on basic footwork, hand wraps, kicks etc etc because im lazy
but that should be a good start. also if ive made a mistake someone correct me