HTTPS Client Only

Hello.
I'm starting now in WEB programming, and I came across a question before the site that I'm developing to go on the air.

As everyone knows, security is essential and extremely important for any system.
My system is hosted on KingHost. They offer the SSL certificate of Let's Encrypt and automatically install it on the site.

My question is this: Having the SSL certificate configured only on the client side, will all the information exchanged between client and server be secure? Or would it be necessary to have the SSL certificate set up at both ends?

I'm sorry for the question, but I'm really starting with this kind of development.