Go to PayPal's Developer Website, sign in with your PayPal account, click "Applications" then "Sandbox Accounts" and create a new "Business" account. Once the account is created, click on the triangle next to its email address, then "Profile". The "API Credentials" tab will provide your API credentials (probably). If this tab is blank, try refreshing the page.
You will also need a "Personal" account to test the transactions on your site. Create this in the same way, finding the account information under "Profile" as well. You may need to set a password in order to be able to log in to PayPal's sandbox for this user.