CloudFlare has made this as a default feature for the Free and Pro plans, so you don’t have to do anything. Business and Enterprise customers will need to enable the feature in the Network section on CloudFlare.
In terms of performance, this is an effective solution to speed up your website, moreover, you won’t have to make any change to your code. As HTTP/2 is based on SPDY protocol by Google, you won’t see much difference if your site is already using SPDY. If you haven’t enabled SPDY, you will see a dramatically faster loading time. Notice that CloudFlare supports HTTP/2 over TLS only, so you will need to enable the free CloudFlare’s Universal SSL at the very least.
Basically, HTTP/2 will speed up your websites with some benefits like Connection multiplexing (multiple resources can be retrieved over the same TCP connection), stream dependencies, HTTP header compression or server push.