First, find the form you want to customize by clicking "Transactions"-->"Transaction Forms" and clicking the "Edit" button on the form that needs editing:
To change the content of the email that is sent as a receipt to all donors, click onto the "Notifications" tab, and scroll down to the "Receipt Email Content" box. If this is left blank, a default message will be sent, with transaction details.
To change the "Thank You" message that is seen by everyone after they run a transaction, click on the "Website Settings" tab, and scroll down to the "Website Thank You Content".
Once you've entered in your content, click "Save".