Some of the comments here are bordering on advice, and unwise advice at that. I use "investment" here interchangeably for investing or trading.
Go to your local library and pick up a bunch of different books on different styles of investing. Read them all and find something that clicks with you. Find more books on that style by different authors.
Create a plan. You will need to cover many things including:
1) What do you want to achieve? Must be realistic - do not expect to make 100% return on the first $1000 you throw at the market.
2) What timeframe are you looking at? Years? Months?
3) How much money will you put in any given investment?
4) How will you know when to enter a market?
5) How will you exit the market? If you don't know this before entering then you are gambling.
6) What will you do if you suffer a losing streak? Say 5 or 6 bad investments in a row?
7) How will you handle winnings? Do you reinvest all your money? Will you divide winnings into 3rds - 3rd for you, 3rd to reinvest, 3rd for the taxman?
8) How will you know if your idea is no good?
And here's a free tip. The very worst thing that can happen to you is your first few investments in shares result in a profit. You won't start to ask the right questions until you've experienced losses big enough to hurt.
You might think, "C'mon. All I want to do is make some money in the share market." But if you don't think about these things, then any money you do make will just be good luck. And you'll lose it just as fast by going too big, too quick.
The fact that you are 18 and thinking about this is fantastic. Wish I'd started that young. When I did start, I wish I'd listened to the kind of advice I just gave you. But I didn't. And you won't.
But be open to it after you realise that the markets are not a place to make a quick buck.