Ask HN: Resources to learn model fitting?

I am absolutely new to this space and seeking suggestions

Problem:

I have an array with 12 values, one value for each month( think of these values as spending of a user each month ).

What I want to do is given any two dates, want to know his spending between these two dates

For this I need to learn how to fit models.

Please suggest me where to start resources, I will follow it from there

Thank you