I can relate to your situation very well I'd say.
I'm a guy who's always been a bit more on the feminine side, at first I didn't realize this, but it became very clear to me around the time when I graduated from highschool. My poses were more feminine than the other guys, and I never had the desire to truly become more manly.
I've always felt that men's clothing never truly fit on my body right. It's always felt rather wrong, and I've always had a fascination with cross-dressing. I have only started cross-dressing in the past year, and I'll get to that more later.
I'm attracted to women (and to an extent guys that look like girls), so I can't really relate to being too aware of men, though.
I have embraced my femininity, but I do not feel that there is anything wrong with the way I am. I haven't had surgery or used hormones, because I should be able to be happy with myself the way I am, and there's nothing wrong with being a feminine guy.
For becoming more feminine, I'd say start with finding a way to deal with any excessive body hair if you have any. Look into hair removal methods and determine which suits you the best. Personally, I epilate my legs and arms, and pluck my eyebrows (just to shape them) and mustache, and shave my beard area (in the future I will find a better way to deal with my beard, but for now shaving works for my needs).
If you've never done it before, try growing out your hair! make sure you learn how to care for it properly. The main thing is to not ravage your hair with a towel after bathing, and instead pat it dry, let it air dry, or use a hair dryer on a lower heat setting and without holding it too close to your hair. When combing/brushing it, start at the ends and then work your way up to avoid having problems with knots.
For your first choice in clothing, go for whatever you like! you don't have to go out in public right away or anything. I only cross-dress in private currently (with the exception of short walks), but I have shared pictures with some of my friends. If you are insistent on cross-dressing in public, I would recommend that you don't choose anything too flashy or revealing. Be subtle about it, unless you feel really confident in your femininity.
For where to get your clothes, it's quite hassle free to just order some things online. It can be way to embarrassing to go out and buy clothes by yourself, but if your girlfriend is in on your secret, then you could go shopping with her. I go out shopping with my female friend for clothes sometimes. You could also try thrift stores, you might find some good things there for both your male and female styles, and for cheap.
You could also buy a bra and stuff like that, and stuff it, if you want to give yourself the appearance of having breasts (and uh, if you want to hide a certain thing in between your legs, try tucking it in by using a "gaff").
Oh, one more thing, when choosing your clothes, make sure to pick things that cover up your manly features and give your overall shape more femininity. If your hips/waist are really thin, getting something that flares out around the area, it'll give the illusion that they are bigger. If your shoulders are wide, don't pick something that is tight or puffy around the shoulders, or it makes the shape too prominent or gives the illusion that they are bigger than they are. A looser top, or one that hides the neck area (like a cowl neck sweater) are nice choices.
I said a lot! sorry if I ran my mouth too much.