I have a client that needs SSL to protect online donations, but I have limited experience with how/when to use SSL. 

I understand that in purchasing a certificate that I am assigning that certificate to an entire domain (IP address really). Is there a way to isolate the encryption to only a single page of the website, or should I just go ahead and secure the entire site even though only one page needs it?

Unsure of best practice here. Please advise.