Clothes Make the Man
You have two goals when it comes to clothing: 1. Pass, 2. Avoid getting treated like dirt. Probably you have a narrow chin and hairless cheeks. If you pass, you look young. Looking young is fine, but going from being read as a 30 year old woman to a 17 year old boy can entail a starting loss of status. Therefore, you want to convey not only masculinity, but also age and social status. Simply put: life is easier if you look rich.
Therefore, you want to avoid dressing in a sloppy manner. It will drop your status AND you're less likely to pass. You want to wear clothes that are unambiguously masculine. Nothing too gender neutral.
But if you're on a budjet, aside from hitting sales and "dress for less" type stores, also check out thrift shops.
Shoes & Socks
Men's shoes. No sneakers unless they're very masculine. Clunky high tops will make you look like a teen boy. I recommend men's dress shoes or wing tips, but working boots and the like are also possible. Argyle socks are casual. Black socks are dressy. White socks go with shorts. I wear argyle almost all the time, cuz I think they're manly.
Pants (Trousers for you Brits)
Avoid gender neutral pants. This means that jeans are risky, but if you can find some that are manly, then go ahead. Get pants that hang on your hips rather than your waist. Men's pants are baggier in the crotch and a bit baggier in general. Don't go overly baggy, though. Get something that fits.
Boxers or briefs? Or boxer briefs? It you're going to pack (fun!), you probably want boxer briefs or just briefs to keep your sock ball or whatever in place.
Also, go for a wife beater or other loose-fitting tank top. It may be possible for you to avoid binding.
Button up, collar, cotton, vertical strips. That's the general guideline. Men's shirts have buttons on the right and button holes on the left. You want cotton or some other non-clingy fabric. Vertical stripes are you friend. Plain colors are ok, but not as good. Same with plaid. Avoid horizontal stripes! Vertical stripes make you appear more box-shaped, which is what you want. Also, the tend to have a horizontal section across the back shoulders. This makes your shoulders look broader. This is also what you want.
Tuck you shirt into your hip-hugging pants. Voilà! Your hips disappear! Do a check in the mirror to see whether you want to bind or not.
Get a shirt that actually fits. Men in the US often wear tents that double as shirts. This looks terrible on scrawny guys and on you might look like you've raided the closet of your dad or boyfriend. Get slim fit if you have to.
Sweaters, jackets, etc
I suggest blazers. Button the middle button (and maybe the top button) while standing and unbutton them all when sitting. Never button the bottom button. Blazers are manly and they convey some social status, but, of course, they're not practical for every event. If you need a sweater, get a men's one that's not tight. Avoid sweatshirts as they're too ambiguous.
I want to give a shout-out to the bow tie. I directs eyes upwards to the collar and away from the chest, while neckties do the opposite. However, neckties are also cool.
Hats are very regional. So, what are the dapper men in your area wearing? Look for what guys dressed like you have on their heads. Don't wear it if women also wear it. Unless it's winter and you need to wear a ski cap or something. It's best to avoid suffering for fashion.
I wear a newsboy type cap ('casquette' in French) on cool days and a pork pie on warmer days. Brimmed hats are great, because they double as sun block! I also have a tilley hat, but people tell me it looks silly, alas.
If you live in San Francisco, go get whatever Willy Brown is wearing. That man is a sharp dresser, especially when it comes to hats!
Go find a haircutter that's trustworthy. This often means a gay man. Other people may try to argue with you about your hair, like it's not on your own damn head. This can be easier if it's already really short. If you want to loose longer hair, get your friend to give you a terrible haircut first. (Ok, I've never tried this, but it might help.)
You want something boxy. Longer on the top. Short on the back and sides, blended high.
Gell it up and out, to add to the squareness. Move hair away from your temples. If you're brave, you can take a razor to square up your hairline.
Your sideburns should be square as heck. This is key. If you keep them high, they will probably look better. Trim them so they are a bit above where your hair stops growing like head-hair.
That fuzz that you're so proud of on your cheeks, chin and upper lip is not actually helping you, unless you're able to grow a real beard. Shave it off. Shave your cheeks, your chin, your upper lip, and under your chin, to a couple of centimeters on your neck. You might want to get the fuzz below your ears too, but careful of nicking yourself. Don't shave your lips. (Who would do that?)
Use shaving soap and warm water. Shave with the grain and not against it. Don't do it too often, or your face will hurt. If you just have fuzz, you can do it every 4 days or so. Change the blade after 2 or 3 weeks or if your face feels scraped.
Don't put Rogaine on your face. Don't put fake hair or stubbly makeup on your face, unless you're doing a drag stage show. Facial hair comes from hormones and that's it.
Now let's look at you
My, aren't you a dapper young fellow!