Individuals cannot place order directly in the exchange they have to go thru brokers who in turn place order to the exchange.Order placed in a day will executed after market close ie after 3.30 pm.During long we buy ,short we sell.We can sell share without owing it if traded we have buy the shares before the market close.Stock prices are decide by the seller and buyers it will not affect the monetary of the company.We should technical analysis buy doing we know the entry ,target ,stop loss levels.