|It's important to add an honorific to someone's name, but don't call them an otaku...unless they label themselves one first.|
In Japan, people are addressed by their name and some kind of title. What title depends on who the person is and how well the person addressing that person knows him or her. Not addressing a person with a title is either an insult to that person or a sign that you're extremely close to them.
So here are the titles, in order of least formal to most formal (excluding -tachi).
-chan - used for a young female (usually) that one knows very well. This can also be used as a term of endearment. It can also be used with young males (under ten) or males that one knows very well
-kun - used for males, usually underclassmen.
-san - used for someone who you don't know too well; it can also mean Mr./Mrs./Ms. and usually when you meet someone for the first time, you'd say (Last name)-san. IMPORTANT: It is considered very rude to refer to yourself as -san
-senpai - used by lowerclassmen to upperclassmen (in school, at work)
-sensei - used for teachers, people you respect a lot or who have taught you something, or people with honorable, or respectable jobs (doctor, mangaka (manga artists))
-dono - similar to -sama, which is a title of great respect
-sama - a title of great respect, usually used for lords, ladies, or anyone who demands that much respect
-tachi - means "this person and everyone with them" or it could be used to make a word plural