It depends by the software client you are using.
Every account is a string wich start with "nsec" (thats the equivalent of your precious password, never share it)
Every nostr client let you create a new account generatic a new "nsec" string OR pasting an already existing "nsec" you have into it.
Some nostr client allow to handle multiple account, like #Amethyst
npub its my login and nsec its my password. I got it haha but where's my password?
Check your profile.
It should list your npub publicly and allow you to save your nsec also.
On Nostter, you can't have multiple accounts unless you create another one and login using copy-paste of your nsec every time (this copy-paste is considered very very bad security-wise & recommended login mode is through an extension like nos2x, but extension => only 1 account).
