I hear a noise when I'm braking.
If you hear a squeak noise when you are using one of your brake, it means the corresponding disk got dirty. Simply clean that disk with a cloth - dry or with some alcohol - to make it disappear. Be careful: don't use soap as it would damage the disk.
A little 'clac' noise when you start braking, is totally normal and does not require any particular action.
I hear a noise when I'm riding.
... only when I push harder on the pedals
The tension of your belt is then too low. Follow this tutorial to adjust it to ~40lbs. If you would like a professional to do this maintenance, you can go to any bike shop. We would happily recommend one close to you! Don't hesitate to contact us through our chat.
... that disappears when I stop pedaling
Make sure to retighten your pedals using the pedal wrench you received in the accessory box and to clean your belt & sprockets with a wet cloth.
If you still hear a noise, make sure the belt is correctly aligned following this tutorial.
... that persists even if I stop pedaling
The braking pads might be rubbing against the disk rotor. Disc brakes can be adjusted to make that noise disappear. You will find lots of tutorials explaining how to do it on the web. Here is one example.