First of all feet position and balance is important, left foot forward, right foot back, spread only as wide as your hips (for orthodox fighter, opposite for a southpaw) get up on the balls of your feet so your heels are just off the ground,you should be able to twist and crouch without losing balance, put some chalk on your heals and if you leave marks on the floor 20 pushups hahahaha, getting your heels off the ground helps you rotate your hips when throwing hooks and leg kicks and you put less pressure through your knee's when your legs can rotate on the spot. Try a right leg kick on a bag with your heel up on your left foot then try it with your left foot flat on the ground, you will notice that with your heel up you will be able to rotate your body into the kick, with your heel down your left foot will stick to the ground and you won't be able to get as much power and you will feel your knee twist more, if you miss the kick you could easily blow your knee out, up on your toes more you will just spin around if you miss. Same with hooks, throw a right hook, both of your feet should point more to the left as you rotate and follow through with the punch.
Keep your hands up,left hand at the front about 8/10 inches away from your face, right hand up near the right side of your chin with your palm towards your chin, tuck your right elbow into the side of your rib cage, throw a left jab, when the jab goes forward put your right hand palm facing your opponent up in front of your mouth and nose so you can just see over the top of the glove, this protects your face from a counter jab from your opponent, when throwing a right hand as the right goes forward bring your left up and put it against your left ear, tuck your elbow into your rib cage, this will protect the left side of your body. After a combination of left/right reset to the starting stance and do it again. BREATHING while throwing punches is something that beginers forget to do, the best method I found was to count out loud when punching, left right left right count them out 1 2 3 4, you breath out when you throw punches, when you throw a quick snappy left right you can do that with 1 breath but you still blow out twice but half a breath each punch but thats later on, just do it slow and get the tecnique first, Practise this over and over again until it just happens without you having to think about it and then all you will have to worry about is tecnique in the future as you progress with your skills.
Hope this gives you a starting point.