Basically, you can only buy stocks for more than 15 NP.
Most people buy stocks in bulk - like when I buy stocks, I buy 1000 of each, so I spend 15,000 NP a day buying stocks.
It's really up to you when you sell your stocks. I personally try to sell mine when they reach 30, just because I'm impatient
Someone else will probably explain stocks a whole lot better than me