That's how the habit, and the addiction, begin. If you smoke, it's not too late to make a change. To quit, you must break your addiction to nicotine and your habit of smoking. Your habit is the behavior that goes with your tobacco use, for example, lighting a cigarette when you get out of school. You can quit.
Most people try to quit more than once before they succeed.
So don't give up if you slip. Just don't go overboard and buy a whole pack of cigarettes. Instead, think about why you want to quit. Think about what happened to make you slip. Figure out how you'll handle that situation differently next time.