What college degrees would be most helpful to study?
As Salesians are known for being teachers of children (as St. John Bosco, their founder, was a teacher of young boys), you should go after education degrees. The education component can be your major or a concentration in another degree.
I’d go with education as well!
You may also think about getting a minor in an area of study you’d be interested in teaching!
Congrats on wanting to join the Salesians! I went to a Salesian high school and considered becoming a Salesian for a few years.
Salesians have an extremely long formation process, so rather than getting any degree and then joining, you'd probably be better off if you talked to them now and got their opinion and help on what you should study and things you could do during these years to grow. Although they do sometimes take guys without college degrees, I know a quite a few who were told to wait until after college. But if you get in contact with them now, they can help you discern, you can grow more familiar with the order (the Salesians are very much a family), and they can help you maximize your college years. PM me if you want contact info for the vocations director or other members of the order.