If you are asking yourself this question, chances are you probably need a certificate.
But let’s consider a few things:
1. Does your site have a login page / administration section / registration page?
If it does, you are probably going to need at least a basic certificate to encrypt the login information when it is being submitted to your site.
2. Does your site store any personal information / sensitive information / billing details?
If the answer is yes, then you definitely need a certificate to protect the transmission of this data.
3. Does your site sell anything?
If the answer is yes, then you need to think about what payment methods your accept.
If you only accept Paypal for example and all the purchase transaction occurs on the paypal site, then the information is being secured by their cert, so you may need need a certificate for your site. if on the other hand you accept Credit card payments directly on your site then you should have your own site certificate and make sure that all the payment pages can only be access over https.
If you answered yes to any of these questions, more than likely you need to purchase a certificate for your site.
Check out the SSL Picker for help in choosing one.