Purchase a pin lock or drill a hole and insert a nail through the inside frame and part way through the metal door frame. You can remove the pin or nail but a burglar can't. These windows are difficult to secure because the individual panes can be easily removed. Application of a two-part epoxy resin to each pane of glass will prevent easy removal. Make sure grilles are equipped with a quick release feature for emergency exits.
Most standard windows have a small thumb-turn lock in the center.
Don't rely on these. They can be pried open or easily reached through a broken pane. You can buy special key locks for windows. Keep the keys away from the windows, but make sure everyone in the house knows where to find them in case of emergency. Drill an angled hole through the top frame of the lower window partially into the frame of the upper window.