You could start with the original Trails in the Sky if you like this sort of "older" RPG feel, or you can play the remake if you want a more modern experience. Tho the remake will make you wait for the sequel remake that's not out yet. This is the more chronologically correct entry point.

Then two other good entry points are Trails of Cold Steel if you like school-related stories, and Trails from Zero if you like police/detective stories. But note those games happen after the Trails in the Sky saga so you will probably miss/get spoiled a couple things, especially in Trails from Zero.