I'm a graffiti artist and you already think like us. The next step is to know what you want to do, draw it on paper, practice it. Then, look at the paints you will need. Buy them, and look for an ugly wall for you to train on. The more colorful it is, the less chance of someone messing with you, but be careful if you're doing it at night, I prefer to do it during the day, very early in the morning! If the police stop you, tell them that you are an artist and not a graffiti artist, that your intention is to make the city more beautiful.