Your first trigger, in a scratch org

The setup is half this lesson on purpose. Most “first Apex” tutorials skip it and leave you writing code in production, which is the habit we are here to not form.

The twelve lines

trigger AccountGuard on Account (before insert, before update) {
    for (Account a : Trigger.new) {
        // The same rule you would write as a validation formula —
        // ISBLANK(Phone) && ISPICKVAL(Type, "Customer")
        if (a.Phone == null && a.Type == 'Customer') {
            a.addError('A customer account needs a phone number.');
        }
    }
}

Yes, a validation rule does this in one line, and in real life a validation rule is the right tool for it. We are using a familiar rule precisely so the only new thing on screen is the syntax.

What to notice

addError on the record, not an exception — the platform turns it into the same red banner your validation rules produce. And the loop: even your first trigger handles all the records, because lesson six is going to make a very big deal of that.