Some people might write a whole song and don't know what to call it. And, like most books, or an essay, or a blog article, you want to pick something that is going to intrigue somebody and make them say, "Well, what is that?"
But also, you don't want to go so far in being creative that whatever the title is, it doesn't match what it is that you're saying in the song. So, one thing you wanna do is, like most anything, is create a process of elimination. Pick three things that kind of embody what you want to say, and think of the easiest way to say that thing.
So, if you're rapping about love, you can say "amour"' or, you know, what things come to mind when you think of the title? And write down some variations, and pick the top three. And then again, you can bounce it off of a friend, a partner, just people in the room, kind of like a focus group.
Pick people you can actually run your ideas by, and help them to pick. But, ultimately, it's really about picking a title that, when somebody sees that title, it will intrigue them to try to find out what that song is about. And make sure that that title does not vary too far away from the content of your song. And that's how you pick a title to a rap song.