Hello,
The subject of 'sonnets' as you describe is difficult. Have you chosen the idea of writing a sonnet because you like them, or are you 'assigned' to do one, hinting at the cause, the pressure of having to do one, the pressure coming from outside of you; that is, a university, a school, or even, dare I say it, a commission? I believe it to be your own homework you are describing.
During my own university studies as an English student, including English Language, I came across the words of that wonderful professor, Arthur Marwick, who coined the phrase, 'unwitting testimony'. I wonder if your studies have included references to him? The phrase is used to define, or analyse, something which seemed to hide other things beneath the obvious; a 17th Century English painting, for example, was not just a representative picture, like a photograph, it portrayed symbollic references, often not obvious to those not fluent in the language of such things.
Using the same techniques, I've look beneath your message and find that you seem to assume some things which are not quite correct. What kind of sonnet, for example, is one requiring 15 lines, since they are accepted as 14? Further, you say you have British Lit (universally known by students of English as 'English Literature or, more colloquially as 'English Lit.', a subject dealing with literature, novels, poetry and other written English) but for some reason you use the word, 'their' twice in one sentence, making the typical mistake of mis-spelling, when any English speaking student would know such things and the basic difference of the use of each version; 'there' as opposed to 'their'. Your tutor would be unhappy, if not appalled.
The basic meter of all sonnets in English is iambic pentameter, made up of an 'octave' and a 'sestet'. That is the form of the 'Italian' or 'Petrarchan' sonnet, which, I assume, you have discovered.
Writing any poetry is not just a matter of putting down some words which rhyme if it is required that they rhyme, or putting down some phrases, including the typical cliches, such as your last line, 'two heads...' Poetry is a matter of writing fluidly, with phrases honed to suit a particular style or scheme. It is composed using the elements of emotion, the thoughts of the author or a fictionalised person, the life situation, the observations and interpretations of those, which, hopefully, will be shared by the reader.
Sadly, there is little information in your nine lines which stirs the imagination, even if it was set out like a poem. You say you 'need to decide what the topic would be.' You seem to imply that you don't want to write about something of the past, family issues, break-ups or even deaths, yet you want something 'tender yet tragic'. You have a wealth of topics within those former topics, both tender and tragic, if only you could explore them.
