Follow this guide https://develop.sentry.dev/self-hosted/.
You need to generate the secret key and add it to config. You can use some password generator for that with long string of random characters like this password:
yc;"2YWZ~AKu9=B`[+.fp2rR(>/6Lk}v
To create a user if you did an unattended install like with command bash install.sh –no-user-prompt,
cd to directory with sentry docker-compose file and run
docker-compose run --rm web createuser