I'm grateful that you're discerning a vocation to the priesthood. We certainly need good and holy priests. Bear in mind that a priest's ability to baptize (a form of exorcism in itself!), to consecrate the Eucharist and to forgive sin in the name of the Church, and to preach are the most powerful weapons against the devil ever!
Exorcists have to be validly ordained Catholic priests, because they act in the name of Christ's Church and they need the full power and authority of the Church behind them.
Priests who are exorcists are typically asked by the local Ordinary (Bishop) to function in that role based on their maturity, holiness, judgment, etc. The role of exorcist, from what I understand, is a very difficult one. If you have a call to become one, it will become known when the time is right and your Bishop will call you to that role. At the moment, however, focus on your own growth in love of God and neighbor, and in discerning your vocation to priesthood. With those in place, your ministry will, God willing, be a formidable weapon against the kingdom of darkness.