Keep up with the news on the economy and businesses. Traditional sources are the Wall Street Journal, MarketWatch, Barron's, Investors Business Daily, Yahoo Finance, SeekingAlpha, etc.
Some are free, others are not. But all should have enough free content to get started. Eventually you'll want to take classes in finance they teach things like asset allocation and portfolio management (ie how to divide up your money into different investments).
But at a young age, getting familiar with the "market" and showing genuine interest is important.
There's also a ton of books that are useful. A Random Walk Down Wall Street by Burton Malkiel or any of the Warren Buffett biographies are interesting.