You can toss all your credentials out the door. Every one of your letters of recommendation and pats on the back from professors will do you no good. If you stink at classroom management, you will not be successful in the classroom.
Thankfully, classroom management is not complicated. Though establishing good management habits is quite difficult (old habits die hard), if you just keep a few things in mind and make them a habit, you will be hitting home runs in your classroom in no time.
1) Love and Laughter are Powerful Potions: if students are convinced you care for them, they are much more likely to cooperate with you than if they think you don't. This does not require that you act like their "buddy." That will quickly backfire on you. Rather, learn to take a genuine interest in their lives. Remember their hopes, dreams, and what they are good at. Ask questions about their lives. Smile and laugh often. Show them how humorous and quirky life can be, and they will enjoy coming to your room.
2) Your Door is the Gate to Two Different Worlds: the environment, shape, and decor of your room is tremendously important. No desk mazes, please! The desks should be arranged such that you can get to any student quickly, because it is crucial for you to remain moving. Acting up occurs more often with stationary teachers, so moving around will help keep a cap on disruptions. Moreover, think creatively when decorating your room. Go past a few trite school slogans. The students must see a very clear difference between your room and what lies beyond the door.
3) Master the Art of Non-verbal Kungfu: have you ever "looked" a student back to work? If you have, you probably found it more effective than barking at a student. "Tim, that's the third time I've...if I see you do that again, I'll..." We all know what you'll be saying five minutes later: the exact same thing. Non-verbal skills communicate a tremendous amount to would-be miscreants. Even where your toes point make a difference! Moreover, in an effort to entertain, many students look to draw you out in a verbal back-and-forth. Don't give in to this! Instead, nip it in the bud with your body language.
4) Let Procedures do the Heavy Lifting for you: structure starts immediately upon student entry in your room. In my room, for instance, the first thing students do is grab their notebooks from a table by the door, then they immediately sit down and complete their bell work, which is written on the door. If a student wanders, I put an end to it quickly. Things like this put much "organic" order in your class so that you don't have to be a classroom cop.
5) Quality Instruction is your Gasoline: when students aren't engaged in the lesson, they tune out. This leads to disruptions. Students should be engaged bell to bell, and they need to be active often. If you speak for 30 minutes straight, of course they'll get frustrated quickly. Chunk up the lecture instead: give three minutes of a Power Point, then have students quiz a partner on it. Continue with the presentation, then have them answer a question in pairs. There are many ways to do this. The key is to make them interact with the information often with multi-modalities.
This is merely a taste. *Tools for Teaching* by Fred Jones and *The First Days of School* by Harry Wong go into greater detail. I highly recommend both books. Consistency is key: if you are consistent in implementing these pillars, you won't feel as helpless at the end of the day.