Since your question is quite basic and general, I'll try to answer equally.

1. Hardware: For a single user instance a Pi 3B+ is sufficient. Still, Lemmy can take up some storage space over time because of the images. So make sure you don't take the smallest SD card you have lying around. I assume you know how install an OS and get basic things running.

2. Get a domain; there are many providers out there. Consider using a TLD of your country (e.g. .de, .fr). Domains are usually relatively cheap. You're probably running your Pi at home, so check if you have a static IP address or if you have a dynamic one. First one? Great, go ahead. Second one: Check if your domain provider offers an API to automatically update the DNS record; example provider api.

3. Have a look at the Lemmy administration docs. Depending on your experience, it is relatively easy to setup. Make sure you understand what you're doing, i.e. first get to know Docker for example, then follow the commands. If you don't understand something, just ask or search online. Lemmy is not very complex to operate, so for every part of the deployment you should be able to find information online.

4. Set up port forwarding in your router for ports 80 (HTTP) and 443 (HTTPS). You can find information for your specific router online, but for some routers this cannot be done.

5. Get a SSL certificate for your domain. You can get one for free with Let's Encrypt.

6. Once you have your instance up and running, I would recommend setting it to "private" first. This way you can play around with your instance or reinstall if something goes wrong without having to worry about federation. Once you've federated (communicated with other instances, e.g. by subscribing to communities of other instances), you really shouldn't reinstall!

I hope this helps you with the first steps. Have fun with self-hosting!