Search for a dance class in the style you want to learn. There are less traditional hip-hop dance classes available at most dance studios.
Purchase appropriate gear. You will need good quality shoes that won't scuff the floors, as well as clothing that you can get physical in, be it spandex or sweats. You also might want to pick up a reusable water bottle. Studios can typically recommend the products that work best for their instructors and clients.
Attend every class. Missing even one class can put you significantly behind, plus since you have to pay even when you are not present, it would be a waste of money.
Watch music videos of your favorite artists. This can give you a lot of insight into the kind of moves that are new and popular, and even help you brush up on older technique.
Practice the moves repetitively. Repetition is key, so watch the best videos multiple times and copy the choreography.
Listen to the beat. The most important part of watching the videos and copying the moves is to pay attention to how the dancers keep the beat. Are they following the drums or the lyrics? Are they freestyling? Dancers always pick a beat to groove to, so in a pinch this can be a lifesaver.
Go out with some friends and watch the people on the dance floor. Many people get away with very little movement, so if you don't have the extreme moves down, you still won't stand out.
Practice listening to the beats. This is easier to do in a club that when watching a music video, because in a club the beat is loud enough to feel. So try finding the beat you feel most strongly and just rock either your hips or shoulders to it.
Let loose. A lot of people at the club may not have rock star dance moves, but they have fun anyway because they just dance how they feel. Just allow yourself to have fun and make up your own moves, and you will never be disappointed.