Kayaks come with an adjustable seat so that you can make it balance and fit right. It is worth spending a bit of time playing about with different seat positions to find the right position for you. Generally you want the boat to balanced (trimmed) so there is equal weight distribution over the front and back of the boat. This will allow the kayak to travel through the water with the least resistance and drag.
A good way to test this is to sit in your kayak on the water and get some one to look at your side profile. Ask them a few questions to help you find out how your kayak sits and use this to help guide you in your seat setting and position.
Example questions might be… Is there more of my bow sticking up out of the water or the stern? Is the rail along the side profile of the boat sitting parallel to the water? Is the boat sitting level or titling forwards or back?
The answers to these questions will help tell you if you are sitting too far forward or too far back. Depending on your height and leg length you may or may not be able to adjust where you can sit in the boat.
There is no set right or wrong position as it depends massively on the individual paddler. However ideally you are looking to get the boat evenly balanced.
Use the manufactures guideline to help you understand how to adjust and move the seat.