How can I wire an outside motion detector light?
Your PIR light needs a constant power source, a neutral and a ground. If you would like to "steal" power from an existing interior ceiling light?.. than you would first have to make sure you have constant power into the light box, not just switched power. 
So it depends how your interior light was fed originally (constant power going to the switch first or the light box first.)

If the ceiling light has only two wires(and a ground) going into the box than it's switched power.... not constant. And for that outside light to work the inside light will have to be on... Unless you put the inside light on a motion sensor and skip your wall switch completely....But thats a new bag of worms...

Otherwise....

FIRST TURN OFF THE POWER

If there are two different two wires than things are looking better. At this point you must figure out which wires you need to use. 
To test that as simply as I can state it is you must go into your ceiling light box and open all the wires (remembering what goes with what because you will hook them back together after).
And also open all your wires up in your light switch box,(so no wire nuts on and all the wires are separate except grounds should stay connected in both boxes)
Than with all wires touching no metal ,turn the breaker on. Take your volt meter to each wire individually and a ground. Whenever you get 120V between a wire and ground, thats your hot! It could be in the ceiling light or the light switch box. So check both. Now see the cable that the hot is in....the other wire is your neutral. But you have one more wire in that cable eh... so what one is your neutral?

Well your light fixture is fed with a switched power... and a neutral.... So if you know what the constant power is from the previous test, than to distinguish which one is neutral and which is switched power....simply look to see which one was on your light switch screws...thats the switched power. So if brown was in your light switch box on a screw...than thats the switched. The neutral usually has a wire nut on it in the light switch box and doesn't go on anything.... But the neutral in the ceiling box will be on the light screw.

So you will attach everything back to the way it was before, except on the constant power, neutral and ground in your light box you will attach the 14/2 wire you run out to your PIR. Using wire nuts where you need to add them.