When you log in to your administrative panel go to Modules and click on the Modules option from the menu. You will see all the modules available listed there. Click at the category you wish to filter by and find the Payments and Gateways option. Select the Payment gateway you want to use and install it from the Install button on the right. Once the installation is completed click at the Configure link and set up all the details. Once this is done, you are ready to start using it.
If you wish to install a module that is not listed there, please visit http://addons.prestashop.com where you will find more Prestashop modules for your online store.