Becoming a Navy SEAL is no easy task but is definitely attainable. Some of the things you can do in preparation for it would be to begin training physically (cardio, weights, functional training). The process of becoming a SEAL can be very intense both physically and mentally and the process tends to weed out a lot of people. You can also read up on what you are going to experience while going through the process, there are plenty of books and videos on the subject. Possibly getting in touch with local veteran groups in the area might be able to get you in contact with a former SEAL/Special Forces veteran.
Also understand there is no guarantee that you will become a SEAL, and you will then have to choose another occupation while you are in the military if you are unable to complete the training.