There isn't really a good bunker app yet. You could use NSEC bunker (custodial, you have to share your private keys with them), Amber (never worked for me), or nak. Nak is the only thing that works, but it's confusing if you're not like really into this stuff, and you have to keep it running in a terminal in the background to sign events. This a major area for improvement in the Nostr ecosystem: