One of the mental model that I find useful for prioritising my todo list is Eisenhower matrix named after US president Dwight D. Eisenhower. Dwight D. Eisenhower once said
I have two kinds of problems, the urgent and the important. The urgent are not important, and the important are never urgent.
Eisenhower matrix is a simple decision making tool for organising your tasks. You can use it to find the task you should act on first.