i would like to become an actress?

Seriously?

The boring option, but one that many of us who are now old farts would love to go back and do all over again.....is to work and study as hard as you can while you have the time, freedom and opportunity.

That's the boring option, you're 15 and that will go in one ear and straight out the other, as it did with me when I was 15.

The only thing I will say in favour of study, is the more academically 'rounded' you are when you hit that job market, the much better chance of getting the job that you want, and that includes getting into something like drama school.
In the meantime, go and talk to your drama teacher at school, if you have one. Show your interest, and volunteer yourself for anything going in that department.

When you are a famous actress, you can thank me by getting me a part in one of your films.

I've always wanted to play the grumpy old Concierge in a posh hotel.

Go ahead and study and finish school and then see what happens.
Now you are 15, when you are sweet 16 you might like to do something else.
Being shy is not unusual at the age of 15. Some people are shy the rest of their lives. An actor can easily be shy as long as s/he can act another type.
Many authors are shy but expresses themselves very well writing.
Having a daughter in the field of theatre - not acting - I can tell you that in GB as well as USA about 90% of actors are without work most of the time and 1en % get most of the jobs.
Get a down to earth job before you start acting -

You have to come out of your shell! You have to be able to mix with all kinds of people.

Plan for those wanna-be Teen actresses NOT living in L.A. or New York City: 1. Watch You Tube videos that are popular and produced by teens your age. 2. Study what elements make them popular (over 3 million hits) The key is going to be grabbing interest right away and holding interest. 3. Get a video camera. 4. Start producing your own videos . DO NOT POST THESE until you have them reviewed and critiqued by those you trust. 4. Keep producing until your critics love your content. 5. Now you might be ready to post your videos. 6. Do Not Get side tracked by negative comments by Trolls. 7. Find local Theater companies and voluteer to do anything just to be around actors and directors. 8. Don't give up on your dreams.
You have to become your own Producer, Director and Writer. IF you have any talent and you are very lucky, you might just get noticed. It takes about 10,000,000 you tube hits to get noticed by the Pros. Hard work and dedication will assist the Luck you need.

May I warn you for putting videos on internet.If you also start adding information where you can be found you might be in real trouble. There are too many crooks out there wanting to take advantage of a young person like you.
Also your family and close friends might not be the most neutral persons to judge if you are a good actor. Only contac serious people like teacher at your school, an amateur theater group in your area and later on a theatre school.