I don't understand how 'be' is used as an imperative (eg be good) and not am (am good). It's not the same for go (eg go now).
Yes, of course it's the same -- in both case, you are using the basic form of the verb.
By "basic form" I mean what some grammar books call "the infinitive without 'to'" and others (confusingly) call "the infinitive".
Usually, the basic form just happens to be identical to the first person: "to go" and "I go". The verb "to be" is an exception: the first person singular is "I am".
If you think about it, though, you wouldn't normally order yourself about. You ought to be wondering why we don't use the second person: "Are good". Again, we don't -- we use the basic form. It just so happens that for every single English verb except one, the basic form and the second person just happen to be identical. The exception is "to be", and so that's the only verb where you can tell just by looking at it that we are actually using the basic form.