To write a sonnet, you should first consider what type of sonnet you want to create and think about the subject matter that might work best. There are two major types of sonnets. An Italian sonnet consists of an eight-line octet followed by a six-line sestet. A Shakespearian sonnet usually deals with love and similar emotional themes and consists of three four-line quatrains and a two-line couplet at the end. Once you know what format you want to use, you can write a sonnet about any subject you feel is appropriate and utilize a rhyme scheme that works within the structure of the poem.
A sonnet is a 14-line poem, usually about emotional or philosophical ideas, rather than a poem that tells a definite story. There are two very popular types of sonnets you should consider, though you can always write a sonnet that utilizes any format you prefer. As long as your poem is 14 lines in length, you can technically refer to it as a sonnet, though you should consider a somewhat standardized rhyme scheme and a structure that serves to reinforce the ideas of your sonnet.
One of the two primary forms of sonnets is known as the Italian or Petrarchan sonnet, which has 14 lines broken down into two parts. The first part is usually structured as an octet, which means it has eight lines. This can be written as a single piece or may consist of two quatrains, which are each made up of four lines. Following this is a six-line sestet that completes the poem. If you want to write a sonnet in the Italian form, you might consider a problem or issue that you can bring up in the octet, and then use the sestet to present a solution or resolution to the problem.
You may instead want to write a sonnet in the English or Shakespearean form, which is quite popular and often used to write about love or other emotions. This consists of three quatrains, which are each four lines in length, and often utilize end rhyming within each quatrain. You can use these quatrains to present a three-part argument, or consider three different aspects of one idea. After the quatrains is a couplet consisting of two lines, which usually rhymes and presents an overview or provides completion to the ideas in the sonnet.
Once you consider each type of sonnet, then you can write a sonnet about any subject you find appropriate. Writing the sonnet itself simply consists of writing out your ideas in a poetic format that follows the structure of one of these types of sonnets, or your own structural concept. You can consider optional features, such as the use of a meter like iambic pentameter, though this is not required. Once you write a sonnet, then you should re-read and revise it until it is as complete as you feel is possible.