How can I change my perspective on everything and learn to socialise?

I have so many things that hold me back, and stop me being able to socialise.

a) I prejudge people, and read too much into what they are saying. I get the feeling a group of girls I am meeting up with don't like me, despite not talking much with any of them, for example.
b) I hate putting my opinion or suggestions across, because I am scared that people will belittle or berate them, or even worse, completely disregard them.
c) I hate speaking on the telephone, especially to companies, because I am scared I will say the wrong things, and they'll think I'm stupid, or that they'll ask me questions that I don't know the answers to.

What can I do about a and b mainly, as they are my biggest problems. Thanks if you can help

a) You are too preoccupied with yourself.
up side: you're obviously intelligent and self aware.
down side: this will be a source of continual unhappiness unless you learn how to manage it.
Rule: people don't think about you as much as you think they do. Even if they don't like you is it important??? NO. I think the only way to get past this is expose yourself to more social interactions, think of everything as a learning experience so you'll take good with the bad.

b) Again same problem as a). you lack confidence and are to self-preoccupied. The worst that can happen is you offend people, which is highly unlikely. You need to FORCE yourself to speak-up. Just think "i don't care what people are going to think of me i'm gonna say it anyway". The more you practice the easier it will get.

c) Don't get concerned with being right all the time. Don't worry about your competence. Only people who DON'T question their competence are incompetent.

In short, don't be scared to make a fool of yourself. FACT: Nine out of ten times people either don't even notice what you thought was a social travesty on your behalf, or they forget it almost immediately. Goodluck.

There are some interesting theories explaining how humans often believe others notice them way more than people actually do. The spotlight effect some say. Keep this in mind as you go through your social encounters. Try testing your thoughts and seeing how rational they actually are. Basically try and prove yourself wrong when you find yourself stuck in this situation.

"Everyone is so worried about what others think that nobody ever notices anybody else" obviously that's an exaggeration but i like to take that attitude when I notice myself feeling the way you described.

all the things you mention are something that most people experience, especially if they they are young but, and the 'big bummer is' they are not comfortable talking about them.

the best way to overcome these kinds of emotions is to let yourself 'practise' them. what happens is this, the first time will be agony the next a little bit easier
so if you take that call and manage to do it you may mess up big time but you will find its a bit easier the next time'
edit..
it is unlikely to kill you so some part deep down registers that it isnt as bad as you imagined
if that call produces a good result you will feel pretty good sraight away
all this will build up and you will eventualy have good confidence and self esteem

but then, the next time you go out with your friends and if you manage to put your view forward then that will also be a little easier even though that's the first time you have ever managed it
at some stage you will think what was that all about when i was (fill your age in here) and be happier in yourself

quote "I prejudge people, and read too much into what they are saying"
what in fact you 'are' doing is analysing what they say too much looking for clues about what their reaction is going to be if you were to say (edit..) something. that's turmoil and causing you stress, which makes you feel more self conscious causing yet more stress, a vicious cycle, try and work out (edit..) ways of breaking it. (edit..) counting slow deep breaths in your mind for five or ten times is a good one
hope this helps and best of luck

you will also find it useful to talk over these sorts of feelings with someone you can trust