Two principles I've been teaching my 11 yo about programming:

1. Whenever you change a program, test it.

2. Adjust the size of your changes based on your confidence. Start with small changes. When those work, you can make bigger ones.

These two rules imply that you should begin any new program by solving the smallest subset of the problem you can test.

(Famous example: The first version of GMail, which Paul Buchheit wrote in one day, was read-only and only worked for his email.)