Depending on what region you live in, there are several ways to add funds to your account. The easiest way to add funds is by using a credit card, however, if you do not have access to one; PlayStation Network Cards are available from certain retail stores in several different amounts.
To add funds, select [Account Management] from the [PlayStation Network] menu. On the following screen, select [Transaction Management] > [Manage Wallet] > [Add Funds], and then follow the on-screen instructions to complete the operation. You can also setup your account to automatically add funds to your account if the value falls below a certain value – this is useful if you have a subscription to services.