To many, writing the test first is a requirement of TDD. It’s how I prefer to do it, but I don’t believe it’s a requirement, especially when starting out.
But that doesn’t mean I’m suggesting you go ahead and code away willy nilly and then write all the tests when you’re done. You still need a tight feedback loop. So how do you get that if you aren’t writing the tests first?
Using small steps: write one slice of code. Does it work? Good, now comment it out! Then write a test that will only pass with the code you just wrote.
Now run your test and watch it fail. This is an important step. If you haven’t seen a test fail, you can’t trust that you’re actually testing what you think you’re testing.
Now uncomment your code. Does the test pass? Good. Now you can refactor. Does the test still pass? Good! Now commit and repeat. I sometimes call this comment-driven development. I’m sure I’m the first person to think of it. I’m very clever.
If you stick to this style, eventually you will start to anticipate how to design the code you’re writing so you can easily test it. Then you may decide it’s easier to just go ahead and write the test first. Welcome to the club.