Hi Chaoskidd, sorry nobody has answered. I guess nobody else is using Inmotion (I’m not).
Unfortunately the best way for a beginner to learn is to do it, which has an incredibly steep learning curve at first. You can probably get a Let’s Encrypt certificate, but you’ll have to do a fair bit of reading (and making mistakes and learning from them!)
To give you a basic picture, think of Let’s Encrypt as a CA service, and their client is a separate project. You can use other people’s clients if the official one doesn’t suit your situation. Once you have your certificate, you’ll have to manually add it to your site. Perhaps through some kind of control panel or account portal?
You say you have ssh access - that’s good. It means that a shell client should work for you if the official one doesn’t. Use the official client if you can, since it gets the most community support here, but you’ll need root for that. Chances are whatever client you choose can only provide you a certificate, and can’t automatically configure HTTPS for your site.
If you don’t have root, I’d suggest using NeilPang’s acme.sh client. You can install and run it as a normal user from your home directory with Inmotion. Just ssh into your account and follow the instructions in the link.
Depending on how Inmotion is organised, you’ll have to choose the appropriate authorisation method. For example, I had direct access to the filesystem my website is on, so I used webroot (“Just issue a cert” on the acme.sh page). That places a small, hidden, temporary file on your website so Let’s Encrypt knows the client has control of the domain. It doesn’t matter if your domain and hosting are from different companies.
Standalone requires that you have access to port 80 or 443, which you probably won’t (Inmotion’s webserver will control those ports). I’ve no experience with DNS auth! You should read Let’s Encrypt’s full documentation. Even though it about their official client, it might help you wrap your head around what each authorisation method does.
Once you have your cert, you’ll need to get Inmotion to use it. I have no advice there! Control panel? Account portal? There should be a way.
Good luck! It is a very harsh learning curve, but it’s worth the effort. Don’t give up!