I need to know if I can only use names that are not taken at all (very frustrating), or if there is a way to use names I really want to use and it be okay.
It may depend on the context of your story and the background of your hero. If your hero did not choose a name, the city that saved may have given it to him, so his name may depend on his appearance, his actions, his behavior, what he represents for the city, or his powers if he has any. If your hero chose his name, it may depend mainly in the Backstory of your hero:
The place/Language where he came from can influence in the chosen name. If your hero speaks another language, his name may mean something in his native language.
The meaning of the name in the hero's life, I will give you an example of this: In the manga of "Boku no hero academia", the main character hero name is "Deku" that means Useless in japanese, but then another charanters hives another meaning to his name
Ochaco finds the nickname inspiring and says that "Deku" gives off a feeling of "I can do it" (頑張る でく?), because of its similarity to the word meaning "to be able to do" (できる?), pronounced "dekiru".
- Tribute to another Hero: The hero may take its name from another hero and modify for its custom.
There you have some infuences for your hero name and you can consider them for picking a name for him/her, I hope this helps for your story.