Questions about Bitcoin

Hey I am 18 and I live in NZ. and recently I have gained an interest in this whole thing, so naturally I came here to ask questions. First one is, what exactly is bitcoin, and the sole purpose of it? How can I start investing in it, and why should I invest in it. Second, Where should I buy it, what is the difference between DEX and CEX. How does buying it work? In fact do I just buy it, send it to my wallet and sit on it like an egg buying more each month? What about other coins are they worth my time. The whole buy low sell high thing is that a real thing?

If ANYONE could answer some of these questions I would be so greatly appreciative.

Thanks a Lot