The mobile app is a great way to find available bikes and docks. Once you've downloaded the Citi Bike app, go to the map to see all the stations available to you. Stations that are green contain plenty of bikes, stations that are yellow are running low, and stations that are red are out of bikes entirely.
Use the bike/dock toggle in the upper lefthand corner of the map to switch the view, depending on what you're looking for. If you're logged into your account through the mobile app, the bike/dock toggle will automatically switch to the dock view when you take out a bike so that it will be easy to find a place to return your bike when you're done with your ride.