UpdatedThese are a little more informal, but nonetheless as important as the simple rules.
This is an historical site, and while certain liberties can be, and are being, taken, try to avoid anachronisms. This means: please don't use modern slang. Do your research: what would a viable profession for your character be? How might they behave? What sort of language might they use? What science was around back then, and what writers had written works you'd like to reference?
At this time, no one used 'Monsieur' or 'Madame.' They called each other only 'Citoyen' if name, or 'Citoyenne' if female, as in, 'Citoyan Robespierre' or 'Citoyenne Cabarrùs.'Is it OK to get some of your character history off Wikipedia? Of course! But do try to show the quality of your writing anyway. And think about things like...
Why might it not make sense, in a post-Revolution France railing against the conspicuous consumption of the upper classes, to wear sumptuous furs and velvets?
Common sense, really.
So what did they wear in the late 18th century, you ask?
Well, women were skirts, sometimes with hoops but more likely, if they worked for a living, without; corsets with whalebone boning to keep a straight, upright figure, and both sexes often wore wigs, though again, the working class often went without. Sleeves, interestingly, were not attached to dresses. Also coming into fashion were looser gowns with empire waists and no boning.
The men wore shirts, often with ruffled collars (though again, not too ostentatious); jackets, culottes (or, guess what--long trousers--if they were Sans-Culottes or just laborers), and hose if they did have the aforementioned culottes, which are a kind of buttoned knee breeches. And yes... go to town with tricorn hats if you like.
Oh, and
here is a great site for signatures and ideas of period costume (thanks to Mica for showing this to me).