Usually the ultrasound can give a very good glance around 20 weeks. Sometimes you can find out earlier than that if your baby's open about it. However, there are many women found the ultrasound being inaccurate. Sometimes ultrasounds at different times will give different results too.
So to play it safe, you can get some unisex items and prepare names for both genders.
Anyways, a healthy baby is all that matters. I wish you a happy and healthy pregnancy.
