You are not required to be a gentleman. Citizenship in our broad republic affords you the liberty of indulging in casualness to the point of loutishness. But if you should aspire to that status, these starting points are worth considering.
Item: A gentleman, unless limited by a disability, ties his own neckwear.
Even if he should find a tie confining and uncomfortable (Have you thought of buying shirts with collars of your current neck size?), he does not whine about wearing one. Gentlemen do not whine.
Item: Baseball caps and gimme caps acquired at the feed and grain store are hats. They are not worn in school, in court, at the opera, in church, or at table.
A fedora is suitable for daily wear during fall, winter, and spring; a straw hat—Panama or boater—during the summer months. The trilby, unfortunately, has become signature wear for hipsters.
Item: You have two options: maintain a neatly trimmed beard or mustache, or shave daily. Displaying stubble does not make you resemble Brad Pitt; it makes you look as if you have been on a three-day bender.
Item: The handkerchief peeking out of your jacket pocket has two functions: display and availability to blot a lady's tears. A white handkerchief is always suitable. Colored or patterned pocket squares can look foppish, and a pocket square of the same fabric and color as your necktie makes you look as if your wife or mother buys your clothes for you.The handkerchief in your trouser pocket (or in your sleeve, if you have served in the Royal Navy), is for hygienic purposes.
Item: Trousers should not reveal your ankles. They should break, just slightly, at the tops of your shoes, not collect in folds around your heels like the wrinkles on a bulldog's face.
Item: Pajamas are sleepwear and should therefore be worn to bed. Sweat pants are exercise wear and should therefore be worn in the gym or on the running track. Neither is otherwise appropriate for daily wear. Stick to shirts and trousers.
T-shirts and shorts are boys' apparel.