I agree with what nostromo said and wish to add to that based off of my own life experience (and personality).

My personality tends toward the reserved side of things (I'm an INTJ if that helps at all) and especially so when I'm focused on something; a lot of people have a hard time with me when they first meet me, generally because I'm very intense. I also have another side of me, a really goofy and playful side - I've noticed most people that don't retreat deep into their own psyche's on a regular basis only mesh well with other butterflies - people love quirky and goofy personality, frisky is a good word too. Cultivate your quirky, playful side and people will have an easier time of caring less about your retracted side.

Don't go changing yourself because people don't "get you". Though, there is something to be said for being approachable and fun enough that the kids want to play with you in the sandbox; all you have to do is bring out that playful, fun personality - even if your eccentricities make that "playful" side really odd or strange, people will still respond positively to it.

I had a rough journey going from being a complete recluse and introverted thinker to being the thing I am (a heavy thinker) to also allowing a fun and "I'm no longer afraid of what people think of me" state of mind. I prefer to hang at home and read, work on projects, or hack but I'm right at home when going out with a large group of friends whom aren't anywhere near being geeks or nerds. I'm also comfortable in my skin to interact with strangers, more so actually, than my super-extroverted girlfriend.

International travel/backpacking will also change that aspect of your personality a great deal, going through India alone forced me to make friends and exposed my psyche to so many things I couldn't even begin to outline what it did for me in a simple comment...

Grooming takes time, figure out what social niche you tend to roll with (hipster? geek? alt? fetish? burner? normal? the list goes on) and start looking people who epitomize those sub-cultures and look at how they dress, match things together, &c... It will come with time, effort, and actually going to the mall.

Hygiene (the other half of grooming) is more important than clothing, honestly. Shower every day and wash your armpits, groin, asscrack, and neck with soap and water (those areas in particular because many of your glands that produce body scents and odors are located in those areas). Use soap or body wash that smells good, if you want to feel like you're a refined urban dweller, get some home-made natural products (body salts with eucalyptus essential oils, &c...). Usually after soaping down, I use a sugar or salt scrub with essential oils in it - it makes your skin very smooth and you smell way better than when you use that Axe body wash crap (not to mention the nasty chemicals they put in it).

Keep your hair trimmed, and not just the stuff on your head!!!! This is so important for men. I'm a huge advocate of facial hair, but we start looking like bears when we let it get out of control, so keep it under control. I have a shaped beard, some men have full but trimmed beards. Some men shave it all, do what works for you but keep it sharp. This also goes for underarm hair, I shave mine, some dudes just trim it so they don't have "tufts" oozing out of their arms. I personally keep my arm and leg hair but shave my chest and stomach.

Wear deoderant! Put it on every day right after your shower. Use scented soaps, particularly, around your groin - mine emits a rather potent fragrance and go from "fragrant" to "icky crotch smell" in a matter of a few hot hours; whether you have potential lady action or not, humans have an uncanny ability to pick up on "invisible" or "barely noticeable" scents. When my girlfriend is on her period, more men hit on her (it's noticeable) even though you can't "smell" anything at the conscious level.

Exercise! Get your body in shape, and even build some muscle - it doesn't make you any less intellectual to have a build, it makes you healthier and more capable of using your brain. Not to mention people are naturally better disposed towards a person that takes care of their body.

There are many books out there on fashion, manners, and hygiene too. A book I liked was "Freakin' Fabulous" by Clinton Kelly - I highly recommend it. Also, take pointers from gay men, they really know how to groom themselves well.