Look in thy glass and tell the face thou viewest
Now is the time that face should form another;
Whose fresh repair if now thou not renewest
Thou dost beguile the world, unbless some mother
The speaker begins with an imperative, urging the young man to look at his reflection in the “glass”, meaning mirror in Shakespeare’s time. This reinforces  the idea of the young man’s possible narcissism. 
 The imagery of the glass or mirror to reflect a person’s soul appears again and again in Shakespeare’s sonnets. A good example is  Sonnet 77 , which also opens with a reference to the young man viewing his image in a mirror. 
 That “face should form another” is a concise way of saying that this beautiful boy should have a child who may inherit his beauty. The “face” is an example of  metoyomy , a part of this young man that represents a whole new person and life.
Sonnet 3