There are a few different options available for you to automatically calculate what your shipping charges are going to be when your customers check out. If you plan to use UPS, USPS, and/or Fedex, you can turn on those options under 'Edit Store' -> 'Shipping', by clicking the 'TURN ON' button next to each one. Each calculator drops down to expose a variety of carrier specific options. You can also add your own 'Custom Shipping Rates' which allow you to charge your customers based on the number of items and/or weight of their purchase.
When your customers check out, they will be presented with a list of shipping choices based upon your selections under 'Edit Store' -> 'Shipping'.