Start with No. 1 or No. 4 (D minor).
Each invention presents a single musical idea (a "subject") in one voice, which is then imitated exactly by the second voice. This creates a conversation—a call and response—that demands absolute clarity. For keyboardists, this means one hand does not know what the other is doing. For a euphonium player, this means something entirely different: playing . two part invention euphonium pdf
The International Music Score Library Project provides various historical and modern editions. Start with No