Arm some of yourselves... — Better, Arm from among you (or, from those with you) men for the war. The details of the selection are contained in the next verse.
Related Commentaries of Numbers 31:3
Numbers 31:3
3 And Moses spake unto the people, saying, Arm some of yourselves unto the war, and let them go against the Midianites, and avenge the LORD of Midian.