These are two great poems and I am sure you will find much to talk about. A good place to start though is to consider what obvious similarities and differences there are concerning the theme or message of the poem. Both poems are an invitation to love and feature a male speaker entreating his beloved to embark on a relationship with him. Both also fit into the "carpe diem" school of poetry, in that the speaker urges the woman to "seize the day" and respond to this invitation now, living life to its fullest straight away rather than postponing life until tomorrow.
In "The Passionate Shepherd to His Love," the speaker urges his love to "Come live with me," outlining some of the simple pleasures of life in the countryside. He goes into great detail about the wonderful things he will craft for his love, including slippers with gold buckles and beds of roses, and promises that young shepherd boys will dance and sing for her pleasure every May morning. The poem is one of temptation, as the speaker conveniently ignores the hardships of life in the countryside and focuses on the pastoral beauty of such a life.
In "To His Coy Mistress," the speaker uses hyperbole to describe the way he would like to show his love to his beloved if there was only time to do so. He refers to his "vegetable love" growing "vaster than empires and more slow." However, in one of the most famous lines of this poem, he refers to the ever-present pressure of time:
But at my back I always hear
Time's winged chariot hurrying near;
And yonder all before us lie
Deserts of vast eternity.
Life is brief, the speaker tells his beloved. Then, in the final section of the poem, in a defiant tone, he urges her to extract what pleasures she can from life, because time cannot be made to halt or pause whilst we play at being "coy."
So, whilst there are obvious similarities in theme and message, you might have noticed that "To His Coy Mistress" places far more emphasis on the effects of time, whereas "The Passionate Shepherd to His Love" does not focus on the brevity of time so much. Of course there are other differences, but hopefully this will get you started. Good luck!