I found out with my boys at 14 weeks, and my girls 16 weeks.
There's a blood test that can be done as early as 10 weeks that'll tell you the gender as well as other information.
You can find out the sex at your 18-20 week ultrasound. However there are places that do 3D ultrasounds if you want to find out earlier. I found out with my son when I was 15 weeks. With this one I found out during the anatomy scan ultrasound my doc had done at 20 weeks.