When finding errors in code, what is the best process to do this?

By walking through, either by hand or through a debugger, you will be able to see each step of the code and the changes that occur.

When it comes to human-computer interaction, what is the one thing that should be remembered during all parts of the design process?

You need to remember what your primary and secondary audiences are. If you keep them in mind, then you can take into consideration their needs and desires.

What is the most important thing to remember/learn when it comes to computer science?

You should remember to be thoughtful and think through all permutations of the question.