Generally the positive side of the wire will have a stripe on it (assuming it's a figure-8 type wire, and not a cable made of 2 individual wires in a sheath.) You might want to pick up a cheap multimeter so you can test stuff like this.
500mA should be fine, as that's just the maximum current it can provide. I wouldn't think the trigger port would pull much. One other thing you might want to get a meter for is to test the open circuit voltage of the adapter you pick up. Some are very poorly regulated, and with very light loads may be considerably higher than 12v.