The main thing to do is to dress for you and not someone else. I grew up when the mini skirts came into style but as a Christian, wouldn't wear things that were not modest ~ not easy sometimes since the stores seemed to cater to what was in but I chose nice dresses and shoes or skirts and blouses that worked well together but never anything revealing.
It's understandable to be a little afraid when you make changes because you know others are going to notice and that may be more of what you are afraid of is what others will say. Thankfully I didn't worry about what others would think and stayed with a style that I thought would be pleasing to God and being a person with low self-esteem, that surprises me to look back on all that but there was something within that said I had to do what was good for me and what would make me happy and comfortable so that's what I went with.
To start out, let your hair down one Saturday when you don't have any plans to go anywhere. Get comfortable with it down at home first and then venture to the store with it down. If you decide to get it cut, just get it trimmed about 2 inches so you don't take off too much and can adjust to the new length without feeling self conscious. Don't let the hairdresser talk you into doing anything beyond that or too far out because as one poster said, you may well regret it and then you would be stuck with it.
I love over size shirts because they are very comfortable so understand where you are coming from but my question would be ~ why are you trying to hide who you are? Be proud of who you are but not in a haughty, obnoxious way ~ do it in a sweet, shy, demure way like a lady. Go out and buy a couple of new shirts that are still a little loose but not as big as what you've been buying. That way you can slowly get comfortable with a new style a little at a time as you and as you continue to buy new things, you can slowly work into things that are you size but still modest (no cleavage, not snug or tight in any way, not too short).
For your presentation, buy a nice pair of black slacks and a nice, simple blouse in a pastel color. No ruffles or lace on the shirt and no large, wild designs. Also get you a pair of nice flat dress shoes (or with a 1" heel) so you don't go out of your element too much but still look nice when giving your presentation.
Remember that change can be a good thing if it is handled right and it sounds like you are at the age where insecurities about what others think are bothering you. You might actually be surprised how well this might work out and how much you enjoy dressing up a little as long as you follow modesty and what makes your happy.
Good luck to you sweetie and enjoy getting creative as you learn how to make yourself a 'more' beautiful person and remember that true beauty is what is INSIDE but you do want the outside to compliment that beauty inside. The way you dress will speak about what kind of person you are and what kind of values you have.
This site appears to have some good suggestions but I only glanced through it:
This is an excellent site, written by young people and you can ask questions if you want to and get free advice by Christian young people and interact with others your age. Both my nieces subscribe to this one.