My website hhtps://techrotten.com already has ssl activated? does the pro plan with $20/ month supports ssl? Or would I need to delete my previous ssl and activate it via cloudfare?
Yes. Even the free plan supports SSL.
Keep your current certificate. It gives your server a secure connection. Cloudflare will put their own SSL certificate in front of your domain so your visitors will get a secure connection on their way to your site as they go through the Cloudflare Proxy.