Okay, first of all, try to use correct grammar. It helps in communication.
When taking pictures with a phone, take in these considerations:
1. Don't try to backlight. Phone's cannot compensate for the amount of light coming into the lens and will cause your subject to turn out black. Instead, turn your subject so that the sun is to your back and then take the photo.
2. Get Low or Get High. Taking a photo as your standing regularly will lead to a boring photograph. Use angles, and your own movement to create DIMENSION in your photographs.
3. The More Light, the Better. Taking photos in dim light is not going to be your friend.
4. Ask first. Don't take photos without the Cosplayer's permission. Not only will it lead to a better photograph but you won't be seen as a creep and you will be practicing good etiquette.
5. Posing. When asking someone to pose and they have no clue how to do it, here are a few tips:
Have them balance their weight on the back leg, putting their right foot forward and hands on hips. Direct them so that their hips are at an angle to the lens'. This slims the body and creates a nice shape.
Make sure they are standing up straight. In this internet age, we all slump. It doesn't look good in photographs.
HOPE THAT HELPS!