This is probably one of those things that you either can do very easily, or struggle severely with. This is mostly a problem that I do not face when I start a new story (sorry!) as I will always have specific names that I would love to use in any of them. They are my babies after all (even though some face extremely horrible situations...) but in the end, think of it in terms of what names best suit your character. If they are a warrior, then something like Vikus, Octus, or even Aelin would work (granted it is a fantasy setting). Now, there are still some rule of thumbs when it comes to characters:
1- Never use two names which start with the same alphabet in a story. It just confuses readers generally. This is to be honest, something I try to break as well, but I keep it under tight control (meaning that those characters will never be in the same chapter unless they are alone and having a dialog). If you are still confused, here's an example:
Jack - JordanMike - Micheal and so on.
2- The names must fit the setting and culture. Let's take Game of Thrones for example. If George had named the Dothraki people "Jack Renulds" and "John Adam" it would just push the characters out of their primitive culture, so the named must fit the setting. It is known. Keep in mind that this is entirely dependent on the genre that you are writing as well, having a name such as "Ragnor" in a children story is kinda of an off for me. Just imagine these lines "there once was a man, in the peaceful loving kingdom of joy who used to chase butterflies! His name . . . was Ragnor the Mad. Okay I may be exaggerating here but you get the point.
3- Translate words from different languages. Or better yet, come up with a completely new one! Tolkiens did it, George did it, so why not you as well? Something that I personally use, I try to translate specific latin words and use them as the base for the name. For example Rory is Latin for the Red King, while Mors is Death, so combining the two would give a character that wreaks havoc and breathes torture something like Morus, and attaching a nick name to that would give them that vibe! Think Morus the Black Knight, or the Red King, even the Bringer of Death!
That's all for now, I will probably make another post to explain in depth my own process for naming the characters pretty soon.
Until next time,