It is a joy to grow herbs.  I have enjoyed it very much. Here are some tips for growing spearmint in your garden.  
You will want to sow outdoors in the springtime, in a sunny to partial shady location.  Make sure all danger of frost is past.  Light aids in the germination process, so make sure to barely cover the seeds with a light layer of soil.  You can sow seeds earlier if need be, or if you desire.  8 weeks before the last frost in your area, you can start seeds indoors.
Plant seeds no deeper than 1/4 inch, and space apart aproximately 12-18 inches.  They grow to about 2-3 feet.  They are a perennial plant.   
Harvesting the Spearmint:
     Harvest the leaves as they mature.  If you choose to dry the leaves, the plants can be cut back to about 3 inches above the soil line, JUST before their flowers open. Engery in the plant can be put back into making more and new leaves, as opposed to exerting a lot of energy to make seeds, etc. 
Enjoy!