Do you know if you have root access on the server? Is it a shared server with other customers, or a virtual server dedicated exclusively to you? Are you familiar with command line server administration at all?
If the answer to the first and last questions is "yes", then please take a look over https://certbot.eff.org/ for information on installing Certbot on your server, to directly obtain and install the certificates there. We're happy to help you with any problems you encounter in this process.
If the answer to these questions is "no", the easiest option is probably a web-based client like https://zerossl.com/. This will let you get the certificate and its other associated files inside of your web browser, and you can then provide them to the hosting company.
One thing to bear in mind in this case is that the certificate will expire in 90 days and you'll have to repeat the process (and again every 90 days thereafter). The certificates are always free of charge but you will have to get a new certificate periodically, which some people find annoying or may forget to do (resulting in visitors to their web sites getting an expired certificate error). That's one reason that we recommend automation if possible, either by installing software like Certbot on the server or getting hosting companies to add official support for Let's Encrypt and handle the process for their customers automatically.