What to write about!

I wanted to try writing a sonnet but Im not sure the topic or how to start. Any help?

Pick what ever comes to mind

First, you must decide on what type of sonnet you plan to write with. It could be the famous Shakespearean sonnet, Petrarchan, or if you want to be modern, "the freeverse" sonnet. - Although the latter may defeat the purpose of sonnet writing. Here are examples of Shakespearean and Modern sonnets:

CXXX
by William Shakespeare

My mistress' eyes are nothing like the sun;
Coral is far more red, than her lips red:
If snow be white, why then her breasts are dun;
If hairs be wires, black wires grow on her head.
I have seen roses damasked, red and white,
But no such roses see I in her cheeks;
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
I love to hear her speak, yet well I know
That music hath a far more pleasing sound:
I grant I never saw a goddess go,
My mistress, when she walks, treads on the ground:
And yet by heaven, I think my love as rare,
As any she belied with false compare.

Sonnet
by Billy Collins

All we need is fourteen lines, well, thirteen now,
and after this one just a dozen
to launch a little ship on love's storm-tossed seas,
then only ten more left like rows of beans.
How easily it goes unless you get Elizabethan
and insist the iambic bongos must be played
and rhymes positioned at the ends of lines,
one for every station of the cross.
But hang on here while we make the turn
into the final six where all will be resolved,
where longing and heartache will find an end,
where Laura will tell Petrarch to put down his pen,
take off those crazy medieval tights,
blow out the lights, and come at last to bed.

As you'd notice, most sonnets are actually in the theme of romance. And if you ever plan on using the modern type of sonnet, as simple love poem with the similar mix of the elements of a sonnet may not always work. Like BC's poem, it must carry a unique element to it (satirical or irony) to give offer something new to poetry.

Wtite something that inspires you, something you think about all the time. Look at your surroundings and observe something that makes you think!

Yes the one who wrote the above comment is write....
thoughts are present all around us..