Starting in Stock Market

I'm interested in starting the Stock Market since I heard it's a good way to earn NP over time, and I'm on hiatus pretty often.

When you're looking at the companies, is it better to buy stocks from someone who's in the green that day? I'm aware of the "buy low, sell high" strategy, but should I always be buying 15NP? I know it makes sense to spread your stocks between different companies, but how much is too much?

I'm at least aware of the rough profit that people wait for to sell stocks, but how long has it taken, roughly, for one of your companies to reach that?