You must have a paypal account setup properly to use online giving.
If you have done this, you can now add items for people to give to. if you are not already, log to the backend control panel of your church website located at www.yourdomainname.com/administrator (where yourdomainname.com is your actual domain name) and go to components>donations>custom fields management.
Click on 'type_of_giving'
Add your options in both the 'values' and 'selected values' fields. Make sure not to delete the options already there and add a | after each item you type in.
Click the save button for every new item you add.