For this type of business , You must know its rules and regulations completely then you may stat and deal in this business.
firstly you must know about stock exchange its way of business and its limitations. There are many sellers that they will urge you to sale their shares . Don't go to them they do it for their own incentive. If you want to Start business you read Forex trading books , Web sites etc.
How Points of any Ltd. Company increase?
when a particular company sells its shares in market people depending upon the profitability began to purchase it its shares. if more people wants to buy its shares than its ordinary shares its share value began to increase and its points on Stock Exchange Market increase.
When to buy Shares?
Its depend upon the nature of buyer. Some Buyers buy shares when its value id too low and nobody is buying share of that company. Hoping that she value of the shares will increase in future they buy shares. But some buyers are risk aware they buy when company's share is at top hoping that this value will not down in future.