smtpd(8) is the default MTA.
On a new install, you don't have to do anything.
Upgraders need to check that the sendmail cron job is gone, check /etc/mailer.conf, /etc/rc.conf and /etc/rc.conf.local to see that only one MTA is enabled; also run newaliases to regenerate the alias database.
The legacy sendmail MTA will be available as a package. Yes, there is a /usr/sbin/sendmail binary; it's a symlink to /usr/sbin/mailwrapper.
If you're already using a different MTA, you can go on doing just that.