Conduct research about an urban planning degree. If there's a local university in your area contact them to see if they offer an urban planning program and get some detailed program information.
Decide what kind of urban planner you want to be. You have many options from environmental, housing, international development to a host of others. Take time to do the research necessary to figure out what are the requirements for each urban planning degree and what kind of urban planning is right for you.
Contact the director of the programs you're interested in and see if you can schedule time to talk to them about the degree. Ask details about the program as a way for them to remember you when it comes time for the admissions process.
Apply and get accepted to the urban planning program and university of your choice.
Complete the degree requirements as listed in the graduate student handbook for your program.
Secure an internship in your field of choice. Most programs require an internship requirement but even if your program doesn't, having an urban planning internship under your belt is important to show that you have experience when you graduate and begin looking for work.
Complete a thesis or project, apply for graduation and your urban planning graduate degree will be complete.