How can a 20 yr old start investing in the financial field? Such as bonds, tax liens, stock exchange...etc?

I took a finance class a while ago and can't really remember much, but I have been saving someone money, and in the end, it's not a lot, but I wanna double it somehow. I've heard that Apple is a great stock...a lawyer I know bought 1,000 shares right when Apple was at it's peak...he made a great profit, and I wanna learn how to do that too. How can I?