How to get girls?!

My flatmate recently started seeing 'a friend', they refuse to call it a relationship, but they've been sleeping together for the past two weeks.
This week, its moved from the bedroom into the living room, meaning whenever they are together over here, I can't even walk through the flat to the kitchen for anything incase I walk in on them like a pair of rabbits on heat.

But anyway, this is really depressing me, as girls can seem to click their fingers, and get anyone they want. Even before she got with this girl, both of them have been having regular sex with other people.
Me on the other hand, ive only had sex the once 'for free' in my entire lifetime, and have had to pay for every other time, as no girl will willingly go near me because im overweight. I can no longer afford to pay for sex due to cutbacks in work etc, so while everyone else I know is having regular sex, I am stuck to a life of celibacy!
Even two of the towns most prominant 'fun' girls shall we say who reportedly will sleep with anything were not interested in me.
Now I know to some, this would not bother them, but when I have to listen to my flatmate having sex nightly, this depresses me greatly knowing i'll never get the same, and that im basically a prisoner in my own bedroom.

So just how do I get girls to be interested in me, even for a one night stand if not anything longer?! Is it even possible? Even on nights out, noone is interested no matter how drunk they get. I must be the biggest, most disgusting failiure of a human being alive

Make yourself better. Do some cardio and weight lifting (perhaps focus more on cardio first in order to lose weight and you don't need a gym for any of this). Keep a log of your progress and gain satisfaction from it, Make sure you dress well and your hygiene is good. Go out and get some interests and/or hobbies. Learn an instrument. Meet new people. Practice the art of conversation with people and make more of an effort to be social with people, not just people you think you might like to **** at some point. No girl is going to go for you if all you think of yourself is that you are: "the biggest, most disgusting failure of a human being alive."

You pay? Hit me up.

Where in the UK are you?

That was fast. I'm behind your door.
How so are you overweight?

Right regarding to the suggestions, thank you of course, but here are the problems:
Weight lifting, can't lift anything too heavy due to breaking my wrist earlier this year, so simply dont have the strength for lifting weights on that arm/wrist. As a direct result of the break too, ive not rode a bike in 6 months now.
Apparently I dress geeky. But I have nothing else, nor can I afford to buy anything new when I dont need anything else.
I have interests and hobbies. Sci fi conventions and photography. Things girls generally think are sad and pathetic, and they also cost money.
I try and meet new people and have converstions. People don't want to know me. At all. Anyone.

Basically the only way I can see of ever getting girls, is to be filthy rich. Either directly, or in directly.

Damn thats what that noise behind it is!
How do you mean? As in how much do I weigh? 300lbs (dead on, no joke or exaggeration, was weighed last week). Im 6ft 1, but almost all of it is around my stomach and bum. My legs are quite toned due to constantly on my feet in work 5 days a week, and my arms/face arent very big, just my belly, like a pear.

I cannot go to the gym though!! It is too far away, I do not have the time, nor would I know what to do and would be laughed out!