I've been working with a lot of babydevs/new coders in work over the last year or so, and I've found that people generally do think: Positive. Negative. Regression. In that order, and quite often stopping at the first one.

It's really useful to write your test plan before you write the code because it helps you think of scenarios/code paths you might miss. It's even better if it's something that you can write an automated test for because then you just go, write tests, write code, run tests, fix and refactor, run tests, commit. And the regression testing comes for free for the next person.

TL;DR: TDD, it's awesome!