The sonnet survives because its constraints do real work. Fourteen lines force compression. A turn partway through forces the poem to change its mind about something. Most failed first sonnets fail not because the writer can't count to fourteen, but because they never let the poem turn — it just says one thing, calmly, for the entire length.
A sonnet has fourteen lines and, traditionally, a meter called iambic pentameter — ten syllables per line, alternating unstressed and stressed, roughly the rhythm of natural speech sped up slightly. You don't need to scan every line perfectly on a first attempt; you need the poem to feel like it's moving at a consistent, deliberate pace, not free-floating.
The two most common rhyme structures are the Shakespearean form — three groups of four lines (quatrains) followed by a closing two-line couplet — and the Petrarchan form, which splits into an eight-line section and a six-line section. The Shakespearean form is friendlier for a first attempt because the closing couplet gives you a built-in, satisfying landing point.
Somewhere around line nine (or right at the closing couplet), a sonnet has to change direction — not in topic, but in attitude. A poem about losing someone might spend twelve lines in grief and then turn, in the last two lines, toward something the grief revealed. A poem about love might spend most of its length admiring, then turn toward doubt. Without a turn, a sonnet is just fourteen lines of the same observation, and it will feel static no matter how polished the language is.
Pick one subject you have a complicated, not simple, feeling about. Spend the first eight to ten lines describing it as if your first instinct were the whole truth. Then write a turn — "but," "and yet," "until," or just a hard image that contradicts what came before — and use the last few lines to say the truer, more complicated thing.
Don't worry about perfect meter on your first few attempts. Worry about the turn. A sonnet with a real turn and rough rhythm will move a reader further than a metrically perfect poem that never changes its mind. IARobo's Poetry Studio can generate a sonnet in your chosen mood and theme, and is a useful way to study how the turn lands before you write your own.
Want to put this into practice? Open IARobo and try it in the Novel Generator, Poetry Studio or Character Creator.