The requirements of a traditional Shakespearean Sonnet are:
• fourteen lines (three quatrains/four-line stanzas, one rhyming couplet/two line stanza)
• rhyming scheme of ABAB for the quatrains
• written in iambic pentameter (each line has ten syllables, the stress falling on the second syllable *be/HIND*)
The rest is up to you! Just choose a theme that you like and have at it. Hope this helps. :)