You got to search for something like “how to create a website”, and let me tell you it’s a whole industry. Onion sites work on regular web technology, Tor is just about how the connection is routed.
I’d start with plain HTML + CSS, then JavaScript (search for “HTML crash course”).
Yes, the host machine has to run 24/7 in order for the website to be accessible 24/7. There is hardware that is more suitable to be used as a server. The lightest thing I can recommend are microcomputers, like Raspberry Pi.
You can also consider a VPS (Virtual Private Server), which are machines that you rent and can remotely access, but keep in mind that VPS providers have full access to your machine as well.