Priests these days generally enter the seminary after finishing their undergraduate (bachelor’s) degree, although there are seminaries that accept candidates who have completed high school. The focus of seminary education is on theology and pastoral care.
If you do an online search for “catholic seminary” you will find lots of information, including courses of study.
If you are in high school, I suggest focusing on getting good grades and on growing in prayer and service. Your involvement in your local parish – youth groups, altar servers, visitation to the elderly, helping out with ministries to the poor (St. Vincent de Paul Society, for example), etc. will be helpful in determining whether this is the life to which you are being called. But you don’t have to give up on sports, spending time with friends, even dating. Being an intelligent, virtuous, prayerful, well-rounded person is a great overall preparation for priesthood.