stock trading-when to buy and when to sell

Stock trading happens in both short and long position depending on the share price variation.either you can buy and sell or you can sell and then buy.it is not necessary to own a share before selling .it is enough if you have share before the settlement time .it is the trader who decides the share price end of the day.either it increases or decreases.this price variation also has impact on the company if changed on larger scale.