I am a 24 Y-O male with severe SAD, depression, OCD, and PTSD. I frankly would never have had any girlfriends at all if it weren't for a girl asking ME out at 18 yrs old. Just having that experience, even though it was an exceedingly shallow, short relationship went a long way at showing me that I actually do have some things to offer in a relationship (even though I was 100% completely and totally resigned to the fact that I would never in a million years ever have a relationship with a girl at that point in time).
The scariest thing is indeed asking someone out or being asked out. Most especially the first time. So just realize that once you get over that first hurdle it gets easier, easier as in still very difficult and scary, but easier. Don't worry, that first hurdle is absolutely supposed to be TERRIBLY awkward, it is for most everyone. Just like when we learned to ride a bike we didnt just hop on and start wheelieing down the street, we fell on our ass a few times. That is normal and your goal should be to at least "get on the bike" and fall on your ass a few times, thats how you learn and get more comfortable with it.
Therefore I think the most important thing to do is to just get out on a date or something SOMEHOW. There are of course many ways to meet people, friends, bars, shows, the internet, just walking up and chatting with someone and give them your number, in case they would like to talk some more sometime (that way you are not putting yourself on the spot for instant rejection, which everyone experiences and should not be feared). Not to glorify alcohol use too much but frankly it can be a great tool at helping you overcome that first time hurdle of approaching someone of the opposite sex. Get some beers in you and you'll be doing things you never thought possible, lol. Of course being drunk isn't the best way to always carry yourself but I think it helps to show that opening up is the only way to really make things happen. Of course its more difficult while not drunk but it can help you get a taste of the reward (getting to know someone new and having a good time because you opened up and got past that initial awkward introduction stage) so that you realize things are possible if you just push yourself to take the risk.
You could also make your goal to politely introduce yourself and just talk shallowly about anything with as many guys as you can during a typical day (if you were a guy I would suggest asking for a number but maybe as a girl you should just chat with them and expect them to make a move... I dont know ... give your number if you like them and they are respectful). The biggest factor in meeting someone is opening your mouth and TALKING to people, even though I understand how hard that is for us social phobics.
My view is 15 out of every 20 people probably wont "click" with any one of us and wouldn't make that good a match of a friend or relationship partner anyway, therefore to increase your chances of meeting someone that would be a good potential match for you we have to communicate with as many people as possible. Otherwise, the perfect match for you could be walking by you everyday and he could be the guy who looks like the stuck up jock type which you despise yet once you actually just say "hello" you may realize he is actually the sweet geeky type of dude you can connect with underneath the leather jacket of his, so to speak.
Make your goal to be rejected as many times as you can during the day. I have done that before and it is actually kind of funny and rewarding in a sense if you approach it with the right attitude. Going in with your GOAL being to get rejected while just trying to get to know someone politely makes it pretty hard to 'lose', even though it still of course is going to be naturally somewhat difficult and awkward (the whole point is that it WILL get less so with time). Just walk around and 'throw yourself' at people. By this I mean just engage them with your natural personality and see how it goes. Some interactions you will find to be quite awkward and some you will find you are completely amazingly instantly comfortable. Yes, this is terrifying at first too but that is the whole point, you MUST ENGAGE IN REPEATED EXPOSURES in order to first overcome and then lessen the fear response. I have done this only a couple of days on my campus, been rejected and looked at sort of awkwardly by a few girls... and had others clearly respectful of me by the time I said goodbye who clearly thought I was a creep when I first said "hi". The point is I have grown slightly more comfortable with and learned a bit more about how to better handle myself in such situations as a result.
Lastly I will say to never ever build up in your mind how a situation is supposed to go or how a person is supposed to be or how they are supposed to react to us, or even exactly how we are supposed to or "need" to act. It never will go exactly as we plan and that is just setting ourselves up for disappointment. Like riding a bike, it doesn't matter how many times you go over it in your head, you will never get any closer to being successful without getting on and falling on your ass many times. Make your goal to fall on your ass. I know that is exactly what us social phobics fear most but the fact is that is exactly what you need to do in order to gain confidence.