You need to use a nostr extension, like nos2x (the original by fiatjaf) or Alby.
But on mobile you are out of look, because extension are not supported (exception: use Kiwi Browser).

dtonon

Maybe you could add https://github.com/fiatjaf/window.nostr.js to overcoming the problem of extension on mobile; it is fully NIP-07 compatible, just drop in a single line of code. And it let also *create* a nostr account.
NIP-46 is probably the most promising path for web-nostr integration.

k00b

I got an error while trying to sign up with Nostr.  The error was something like “plugin not found”.