Robinhood allows you to place fractional share orders in either share or dollar amounts. All trades are rounded to the nearest penny. If you place a market order to buy or sell in shares you will need to specify the number of shares you wish to trade. The smallest amount of a share you can buy or sell is 1/1000000. Open the Robinhood mobile trading app and find the stock you want to trade or invest in. In the trading window, click on Trade and then buy or sell. Then you will need to specify whether you want to buy in dollars or shares.