In addition to the comments others have provided. Take a reasonable complex problem and solve it. Now refactor it. Not sure what refactoring is, read Refactorin

In addition to the comments others have provided. Take a reasonable complex problem and solve it. Now refactor it. Not sure what refactoring is, read Refactoring: Improving the Design of Existing Code by Martin fowler. Show your code to others and seek feedback. Always assume that your code can be improved.