Your poem must have fourteen lines. Each line must have ten syllables, and your meter must be iambic pentameter. The rhyme scheme is ababcdcdefefgg.
This may help: http://www.ehow.com/how_2184610_write-shakespearean-sonnet.html
Here is one of mine:
The Commandment
When all of life’s become a field of tares
That chokes the harvest of your fertile years,
And every moment’s thought is filled with cares,
Nay intimations that the harvest nears
So that you fear to plant the next year’s seeds
Lest you be absent when they’re fully grown
Or that they’ll go untended like your needs
That you’ve ignored until the chaff is blown,
Then you must reap before the summer’s gone
And all your stalks are stiff and bare and dry
And fallen from your scepter is the awn
That crowned your crop in better days gone by;
So swing your sickle, yea both sow and reap
And thereby nature’s first commandment keep.
