That's the problem with federated platforms is that sign up is often confusing.
However you can go to a large instance like lemmy.world. sign up is literally just as easy as creating an account on any other site. At which point you don't need to sign up for other instances unless you really want to or if they are defederated, as long as the instance you created the account on is federated with the instance you want to interact with you can comment there with your account from the other instance.
The boost Mobile app works pretty good too