Shouldest know his will - His will in the plan of salvation, and in regard to your future life.
And see that Just One - The Messiah. See the notes on Acts 3:14. As Paul was to be an apostle, and as it was the special office of an apostle to bear witness to the person and deeds of the Lord Jesus (see the notes on Acts 1:21-22), it was necessary that he should see him, that thus he might be a competent witness of his resurrection.
Shouldest hear the voice of his mouth - Shouldst hear and obey his commands.