Change your W-4 now to remove the exempt status.
And at the age of 19 it is highly likely you are still a dependent of someone unless you can prove you paid for more than 1/2 of you living expenses from earned income like wages.
There is a very good worksheet to help you determine how much support you provide. It is on page 15 of IRS Pub. 501
<a rel="nofollow" target="_blank" href="
http://www.irs.gov/pub/irs-pdf/p501.pdf">http://www.irs.gov/pub/irs-pdf/p501.pdf</a>
