Decide on the type of sonnet you want to write. The Shakespearean sonnet is always good to start with, because of its simple rhyming scheme and structure. Shakespearean sonnets consist of an ABAB rhyming scheme, which means that every second line should rhyme. The 14 lines within a Shakespearean sonnet are always separated into three quatrains and a couplet.
Determine a topic for your poem, the nature of your poem, and the purpose you would like it to serve. Sonnets about memorable experiences and people work well. For example, if you want to write a sonnet to your wife for your first anniversary, the poem needs to make your wife feel that she is loved and appreciated. Think about metaphors and other literary devices to add to the impact of the sonnet. Make the main theme a metaphorical comparison, determined by your subject. For example, you might compare your loved one to a sunny day.
Decide on how you will make your poem and your metaphor fit with the common structure of the Shakespearean sonnet -- three quatrains and a couplet. A quatrain consists of four lines, and the first quatrain in any sonnet is used to introduce the subject and metaphor. The second quatrain builds on the metaphor that is established in the first, and the final quatrain usually serves to create a conflict of some kind. The couplet -- the final two lines -- is used to conclude the poem and wrap it up nicely.