I was like you at 17, I could read out loud in class sat at my desk but ask me to stand up in front of a group and talk and I suddenly got very nervous. I hated speaking on the phone too.
All I can really say is, it comes with practice, and with facing your fear head on. Going to uni helped me loads in terms of confidence and speaking to people I didn't know or on the phone because I realised that because I was living away from home, if I didn't sort things out for myself, no-one would, so I just had to do it. Now, at 22, that kind of thing doesn't phase me much at all anymore. Working in customer-facing jobs (mainly as a waitress) to earn a bit of money as a student helped too; you have to deal with the person in front of you or they're not going to be happy, and you have to do it with a smile too. And then last year I went for interview for a job I really wanted and got it, because I'd prepared really well and I knew I had to come across confidently. Because I really wanted it, I found confidence from somewhere!
Public speaking is a more difficult one. I still get nervous when giving presentations, but getting good marks for them has boosted my confidence and I get a little less nervous each time. Practising what I'm going to say and doing a lot of research really helps, and having it written down in front of me just in case I am overcome with nerves and forget. I try not to read from a script as it can sound forced but it's nice to have detailed notes as an insurance if you like. Practice in front of a friend or relative who can give you feedback on your presentation style.
It's good that you've recognised your issue with it, as in many careers it's not something you can escape from unfortunately. If you're getting extremely anxious about it to the point where it's really affecting your life overall, and you're being assessed on public speaking, when you start uni, look into what the student welfare service can offer. I know at my uni if you did have severe anxiety around giving presentations they could arrange for you to do any assessed ones separately in an office to one tutor.