OK. The first steps are the confusing bits. Keep at it and you'll get there. Start by heading to the Mastodon site and creating an account. You'll instantly be asked to select a server from the list the site shows you. As you'll see, these tend to be broken down into areas of interest, so you'll have to spend some time picking the one that you think suits you best. (If you want some help with that, a separate website called Instances can help you narrow down your options.) Once you've selected a server, it's fairly self-explanatory and you should be set up within a few minutes. The service then runs on your browser or on a mobile phone app, just as Twitter does.