Acquaintances, coworkers, and friends address one another by his/her last name + さん. This is the most generic address. One's superiors (people older or higher in social ranking) are addressed by his/her last name + title (if the title is known) or last name + さん.

If the speaker is close to the addressee, the use of first names is allowed. For example, friends and peers may address one another by first name + さん. To address young children, the speaker can use first name + ちゃん for girls and first name + くん／ちゃん for boys. Unless you have a very close relationship with the addressee (e.g., very close friends and peers), addressing him/her directly without a title can be very rude.