Start giving a shit about what you think of you. If that's hard, it's because you've not really thought about it before. Once you've got your own back, nobody else really matters.
Also talk to people. Talk to them all the time. Standing in line at the store. Passing by on the street. Just make an observation about something to start the conversation. Compliment someone. Over time you'll get two things from this. First, you'll realize that everyone is just a person like you (like, the subconscious part of your brain will get the message). Second, you'll get a bunch of little rejections (weird looks, things like that) over time in a scenario where you basically have no skin in the game. You'll no longer care about rejection and you'll realize that people rejecting you almost never has to do with you - it has to do with them.
Yeah, realize now that unless you're making daily headlines for your misdeeds or any gossip you may be generating is actively and visibly impeding your life, even those that gossip about you will be gossiping about something/one else tomorrow. In the grand scheme of things, as far as the world at large is concerned, you're meaningless. Not to say you are meaningless, you're quite probably a most lovely person, but if you take the number of people who know you, that you care about and care what they think, divide that by the number of people in the world and the resulting number relates somehow to the amount of significance you have. Given that lack of significance, which is remarkably unfair considering how awesome you are, does it really matter what you do as long as it doesn't land you in prison or impede your freedom? I mean, if nobody gives a shit, why should you waste time caring about what they think? It doesn't serve to make you happy in any way...
Do whatever you think is more 'scary/make people laugh at you'. Then repeat, and again, until you no longer care.
I'm 33, haven't cared in a long time. Just do stupid shit and own it, with time, it is still stupid shit, but you won't feel embarrassed by it (I live in a posh area, my kid goes to a posh school, I wear 15 year old tshirts (that lost all their color due to washing), no shoes/socks, use shorts 90+% of the time)
edit: And interestingly enough, even though I'm married and with little interest in that, the amount of female attention I get over the ones that care is amazing (and no, I'm not good looking, 5'6, bald earlier in life, long beard and a few scars in my face)
I have found meditation helpful. It has enabled me to treat my feelings more like weather, to put some distance between stimulus and response. If you spend enough time sitting quietly while letting your thoughts come and go, you begin to take them less seriously.
For me it's in some ways similar to the part in the debugging or performance tuning cycle where you're just watching things happen, letting logs and metrics scroll by on your screen. Externally, you look like you're not really doing anything. But what you're doing is noticing relationships, reoccurances, rhythms. At some point you say, "That's odd. When X happens, Y follows. I wonder why that is?" And that's where the magic begins.
Pay attention to everything that is apparently mundane and ordinary. Don't dismiss it as normal or uninteresting. The more different things you can appreciate in this way, the easier it is to let go of the "surface stuff" and be who you are inclined to be naturally, as over time your values will adjust to make room for the mundane and it correspondingly becomes easier to see what you really care about. If you cling to a narrow value system then you never hit the point where it coheres into a life you feel in control of - and that will really leave you adrift when you're older and all the opportunities have passed!
It depends on what is refraining you from achieving it. Is it because you aren't confident enough? is it because you haven't achieve your most important goals in your life? is it because you are introvert? the most you understand yourself, the easier it will be to solve it.
