Answer 1

There is no completely systematic approach to solving a problem or designing an algorithm. If we had one, that systematic approach would be an algorithm to solve every computationally solvable problem.

What realistically works is continued practice solving simple problems through to more complex ones.

Aside from practice, you may want to read about computational complexity and simple data structures like linked lists, trees, hash tables. The goal with this reading would be to get a general understanding of how to relate code and its inputs with the RAM or CPU time needed to complete its job. These topics become important when you're pushed to improve performance of your code. They're also taught in most university computer science programs and might come up in job interview questions.