Yes, amethyst is a client for android. If you want to use nostr from the computer, you can try for example coracle (nostrapps.com/coracle)

There are also clients for apple.

To use a client you'll need your nsec. You should never tell your nsec. To avoid inserting your nsec everywhere, they have also created signers, like Amber (nostrapps.com/amber) for android and nos2x (nostrapps.com/nos2x) as an extension on your computer.

Ideally, you should never insert your nsec in a client, only in signers.