There are two types of ways to buy shares: a Market order and a Limit order.
Market Orders are typically used when the Market is open (between 10am and 4pm AEST during the week) and shareholders want their order to be processed quickly. The share price when you submit a Market order is based on the current market price.
Limit Orders let shareholders select the price they want to pay for the order. Limit Orders may take longer to be fulfilled and will usually expire within 7 days. If you want your order to be fulfilled quickly, Market Orders are the most common.