Step one, realize you're not banksy.
Step two, get an idea
Step three, get paint.
Step four, go paint
There's no real way to do street art because of all the different methods and kinds of graffiti
Its expressing oneself in a form of art
Anyways you're going to want to find a surface to put your art on, different kinds of graffiti art for different surfaces, think about it, the texture and the size are going to be in play for an aerosol artist. Some artist don't use paint, for example the graffiti artist “fafi” uses flowers she puts up chicken wire on a fence and sticks the stems of the flowers through it, There is also an artist called invader who uses tiles. Second thing you're going to want to do is get your idea and your colors. You should probably draw what you are going to do on paper first. If you're not into it being illegal, talk to someone that runs the town or, someone who owns the building, but if you don't want to deal with that stuff I’m not going to be the one to take away creativity. Clean the surfaces you are painting on, more well-known artist prefer to use a power washer. After this you are going to want to lay down a coat of paint so your art or tag can pop out more. The rest is all you.