There stood by me - There appeared to me.
The angel of God - The messages of God were often communicated by angels. See Hebrews 1:14. This does not mean that there was any particular angel, but simply an angel.
Whose I am - Of the God to whom I belong. This is an expression of Paul’s entire devotedness to him.
Whom I serve - In the gospel. To whom and to whose cause I am entirely devoted.