This is where the ship moves least. If nausea still comes your way then lie down and keep your eyes closed. In a plane Ask for a seat over the wings. This is where the plane is most stable. Tilt your seat back, rest your head on the headrest, and close your eyes.
Music helps some people to relax and takes their mind of feeling unwell.
Some people know they are going to be sick whatever they do, particularly when taking off or landing. If this is the case then ask for a seat near the toilets. Keeping still with the eyes closed helps. Many different anti-sickness medicines are available from the pharmacist. These should be taken before travel according to the instructions so they have a chance to be absorbed into the body.