How do you learn to hip hop dance?

Many dance studios will offer hip hop classes.

As will gyms- these classes will probably be very aerobic.

And colleges/universities will often offer classes in hip hop.

It helps to have danced another style before you start hip hop.  ie jazz

Not completely necessary though.

There are distinct styles that fall under the broader category of "hip hop"

Popping- the muscles "pop" contracting and relaxing

Locking- fast and distinct hand movements

Breaking- commonly known as "break dancing"

Breaking is about the fancy moves you do in the "breaks" during the music

Many dance studios, gyms, and colleges offer schedules online. 

So you can do a quick search with google and see the schedule and call them if you have more questions.

A gym may require membership.

Usually you can wear athletic shoes, but sometimes the must only be used in the studio. 

Black soles are often not allowed since they will make marks on the floors, especially for dance studios.

For classes you can wear athletic clothing or baggy clothing.

You will likely sweat, since you move a lot in hip hop.

Individual classes may cost $10 and higher per session.  If you go to a whole series it can be less.