If you have forms, a login, or an e-commerce website that collects data from your users, contact your web host to get an SSL certificate. This will encrypt user information, increasing the security of your site. Your url will start with https assuring your viewers that their information will remain private. Soon Google may give lower rankings or penalize un-secure sites.
If you have a portfolio website or blog that does not sell directly to users, you are OK without an HTTPS SSL certificate.
I encourage you to learn more about this topic in greater detail.