Excellent questions! Asking such questions indicates something special about yourself. The desire to learn and adjust your beliefs will increase your chance of success in your life. I would use a wide variety of authors to increase your education. Myself I prefer Dave Ramsey to Clark Howard, but I think Clark is very good.
The first thing you should focus on is learning how to do and live by a budget. Often times, adults will assume that you are on a budget because you are broke. It happens with my friends and my youngest child is older than you. Nothing could be further from the truth. A budget is simply a plan on how you will spend your income so you don't run out of money before you run out of month.
Along with budgeting I would also focus on goal setting. This is the type of "investing" you should be doing at your age. For example if your primary goal was to become an engineer, my recommendation is to hold off buying stocks/mutual funds and using your current income to get through school with little or no student loans.
Another example might be to open your own HVAC business. Your best bet might be to learn the trade, working for someone else, and take night classes for business management.
Most 18 year olds have very little earning power. Your focus at this point should be increasing your income and learning how to manage the income you have.
Please keep in mind that most debt is bad. It robs you of your income which is your greatest wealth building tool. Car loans and credit card debt is just plain stupid. Often times a business case can be made for reasonable student loans. However, why not challenge yourself to take none. How much further ahead could you be if you graduate, with a degree, when your peers are strapped with a 40K loan?
Keep up the good work and keep asking questions.