How to make people care about your protagonist?

Or rather, how to make a protagonist that people care about. I have an idea for a story that I want to write, as a 'fic' that will just be published online for me to explore writing and tell a story that I have in mind! The character is already formed, as is the general outline of the story, all I have to do is plan & write it. However, I would like to think there are people reading and enjoying it, otherwise there's probably little point posting/'publishing' it online. But to make people want to come back for the next chapter/instalment, it has to be engaging but specifically the protagonist has to be somebody they care about and want to see the progression/fate of.

So my question is, what things can evoke that in readers? What pulls them into a character, make them want to learn more about them, want to see where their story leads and root for them. I really want to take any and all advice into consideration!