Help me with stock question

I was recently awarded stock in my company as an equity award.

It was 15k total, vesting 1/3 a year for the next 3 years.

I was awarded the stock in March. My first vesting date is in December.

The stock has literally doubled in value since then and is now approaching 30k. Assuming this held, I would pull out 10k for my first third.

What taxes will I pay on that? I'm assuming capital gains? Do I have to pay taxes on the original 10k as well? Or only the gains? My bracket is the 25% I believe