The two places we recommend putting the "Give Online" button are:
1. The homepage. This is the first page visitors and potential givers will see, so it's important to put the button somewhere easy to find. If you can include the button somewhere in your site template so that it appears on every page, that will make it the most convenient for givers.
2. The giving/stewardship page. This is a page that some churches have that explain why giving is important, what gifts are used for, and the different options someone has to give to your church. If you have this page, you should definitely put the online giving button here, and describe the benefits of giving online to educate your potential supporters.
Here's a great example: http://victoryofthelamb.com/online-giving/