There are 3 cases in which sins are forgiven.
1. If you're unconscious.
2. If something is forced upon you.
3. If you do it by mistake - didn't know it previously.
Well sister, you can tell your parents to buy you long clothes, long as in, long pants and shirts (convert to what the dress is called for women, sorry, I don't really know the names of women dresses). And for the head covering, you can buy a long scarf or such type if you can't find something similar to a hijab - khimar - jilbab, (all mean same).
And I believe any sister can help you better, am sure they will Inshallah.
I will just say in short the criterias of dressing up in Islam.
There are 6. First one is different for male and female, rest 5 are same.
1. Extent: navel to knees at least for males.
For females, full body covered, only face and hands up to the wrists can be shown (though there are scholars who say even those should be covered).
The rest 5 are same for male and female.
2. The dress shouldn't be tight enough so it reveals the shape of the body.
3. The dress should not be glamorous enough so it attracts the opposite gender.
4. The dress should not be transparent so that you can see through.
5. The male shouldn't wear the dress of a female and a female shouldn't wear the dress of a male.
6. The dress shouldn't contain shirk, as in, signs of the unbelievers.
So that was in short of how one dresses up in Islam.
I hope this helps you when you go to buy the dresses.
If you keep these rules in mind, Inshallah, Inshallah, you can get the dress which Islam prescribes and may Allah help you sister and welcome to this perfect way of life, welcome to Islam, may Allah grant you Jannah (Paradise) Inshallah.
Should you need any more clarifications, please feel free to ask.
God Bless You.