Know and respect the history of the art. Graffiti is a hobby with a long history. Look at early hip-hop and the first writers (people who write graffiti), such as Taki in NY. The worst thing to experienced writers are "toys" making fools of themselves and destroying the scene for everyone else.
Pick a name. Practice drawing letters and pick some that you like drawing. Do not try to get too crazy with extensions and bending at first. Take it slow and make everything as clean as possible. It is very helpful at this point to find inspiration from other pictures you see. However, in the future, you will want to develop your own, unique style. A very important aspect to creating a good style with your name is flow. Keep characteristics of letters common throughout the rest of your name. Style is everything.
Carry a sketchbook and pencil with you wherever you go. Practice your craft as much as possible. As you come up with new ideas for tags and designs, sketch them in your book and develop the concept by trying out different variations. If possible, carry pencils of a few different colors. Eventually, you can move to using permanent marker.
Work with your cans. Buy a piece of plywood from Home Depot to practice on. At this point, colors and styles should be simple. Practice your can control and your techniques for painting and steps for putting together a clean "piece."
Be smart. Many cities also have "permission" or "legal" walls specified for graffiti art. Legal graffiti is often more respected than street art. You have unlimited time and freedom to do what you please. Compare legal walls to quick throw-ups found on the street. You will find that legal walls are intricate and aesthetic. Street art is just trying to be seen. Legal graffiti art is more remembered and respected by those who do not completely understand the artform.
Try to meet experienced writers who can help you learn the ropes. (However, be cautious; you do not want to come off like an undercover officer.) Show your appreciation for their work and their skills.
Don't get frustrated with lettering, it takes time.
Wherever you write, keep in mind that you are representing the entire graffiti culture anytime anyone else sees it. People usually don't differentiate between writers, they just know that somebody destroyed their property. Stay away from privately-owned businesses, worship areas, high profile business buildings. The experienced writers nowadays tend to stick to low-profile spots such as tunnels, abandoned buildings, and, if so desired, legal spots.
It's a lot easier to use stencils. Buy some sticky paper and put it in your printer. Use Photoshop or any other graphics program to make the paint layers. Print them.