when people look to invest, the main goal is to earn profit. no one wants to invest in a company which is experiencing losses. the simplest way to understand what the company's profit/loss is would be to look at its financial report, ie. the earnings. when earnings is bullish, the stocks generally rise in price. when earnings are bearish, the stocks typically dip in price. however, not all stocks follow this rule. there are some stocks which historically did not rise when the earnings of the company rose. in these cases, it is likely because of the consumer's and market's attitude towards the stock. perhaps they feel that the stock is not worth its market value (overvalued). it goes to show that one should know your stock before purchasing. know which is speculative which is fundamental.

I decide to buy when I believe in the stock. it's important to understand the workings of the company and what they dabble in. if I believe in their technology, I will buy their stock as an investment and support for their dealings.
as for selling, it depends on what I want to achieve. there are some stocks which I buy for profit taking, hence I typically sell when I feel I have made some profit. it is never wrong to take profit at any point in time. the most important thing is not to fear missing out, and not to sell when there is a dip. long term studies have shown that in general, long term investors do profit. hence invest what you can afford to lose.