Adding of "o" means that the name is Esperantized (turned into an Esperanto word), and the "o" means a noun. For example, Petro for Peter, Aleksandro for Alexander, Paŭlo for Paul. For your name, it might be "Deriko".
It can, but it doesn't have to be used. You can use the native form of the name, an "Esperantoized" name using -o, or "Esperantoized" with -a. The use of -a is because in many languages the choice of -o/-a is male/female. So if your name is Maria, you wouldn't want to use Mario because it is a "male" name. For example, in the course you'll see the name Sofia used.