Once you've registered for an online trading account, navigate to your dashboard to manage your account. From there, you should be able to look up various stocks and other assets. Once you've found a stock you want to buy, enter the quantity and your bid price and submit your trade.
Selling stocks is very similar — you'll find your assets in your portfolio and should be able to select a stock to manage your shares. If you want to sell, you can enter the amount and the ask price, then submit the transaction.