We've identified the pirate vessel and confirmed its identity. At its current speed, it is unlikely we will get in range to use guns on this target. Select your patrol boat, press F1 to order an automatic attack and then click on the pirate vessel. Your patrol boat will automatically move towards the pirate vessel and engage with missiles or guns as appropriate when it comes into weapons range.

Alternately you can handle the intercept and weapons launch manually: Plot an intercept course using the course plot tool (F3), measuring distances and angles as necessary using the measuring tool (Ctrl+D) and adjusting your speed with the speed and altitude settings window (F2). To allocate weapons, press Ctrl+F1 to manually assign weapons and click on the target vessel.