Choose a topic. Love, romance, life and nature are all traditional choices for sonnets, but, unlike some forms of poetry, any topic is fair game. Sonnets need not be serious in tone. You may write a sonnet about your struggles with learning physics if you choose.
Choose a form (rhyme scheme.) Rhyme schemes are written using letters of the alphabet to represent each line. All lines that are assigned the same letter should rhyme with one another. There are three main forms of sonnets. Shakespearean sonnets (sometimes called English sonnets) use the rhyme scheme "a b a b c d c d e f e f g g." The basic rhyme scheme for an Italian sonnet is "a b b a a b b a c d c d c d", though this form also offers some legitimate variations in the last six lines. The Spenserian sonnet uses the scheme "a b a b b c b c c d c d e e."
Write your poem. Remember the rhyme and meter requirements. The first few may take lots of time and practice, but be patient and stick with it. Once you master the technique, the sonnet is considered one of the easiest rhyming poems to write.