Welcome to BrickPicker.
Wow, it's really hard to cram all there is to know about LEGO investing, but I'll try to get in as much in as possible, as well as give you a few resources to get yourself off the ground...
LEGO investing is very similar to investing of any kind. You use the basic principle of "Buy low, sell high", but instead of buying stocks, precious metals, or other more commonly invested commodities, you are investing in LEGO bricks. Like most quite a few things, LEGO's appreciate as time progresses. The main reason for the inflation of prices of sets is because the LEGO Company, or TLG, discontinues the production of sets after a period of time, usually about 2-3 years, though there are exceptions, such as the 10188 Death Star.
In order for the price of a set to increase, you need to choose "good" investments, which means that there has to be demand for the sets after it retires. Ask yourself whether people will want to buy the set 2 years, 3years, or whenever you plan on selling. If you think it will be in high enough demand to increase the price, then there's a decent chance that other people agree with you.
I am also a teen, so welcome to the club! Teen investors of LEGO are a very small group, primarily because teens are either going out of LEGO, or even if they retain some interest in LEGO products, they don