This informal and short guide will help you to take good photograph's of your items, which will help to sell them! :)
Ensure the background is plain and not distracting - a white sheet behind large items works well, and a white peice of paper behind small items works well too.
It is useful to have good, even lighting (outside or inside with the lights on) falling onto your object, so you don't have to use a flash - which can reduce the detail of your object.
Make sure that you have all the important parts of your item in the photo, for example - does your necklace include a box? make sure you photograph it! and remember to photograph the object and the object only - other objects (the cat, the dog, your foot) will be distracting! Make sure that the space around your object isnt too big either - you want the viewer to be able to see the object clearly, particuarly when the photograph is small in the gallery.
Photograph's taken on your phone will generaly have poor quality, it is definatly worth investing in a digital compact camera! I suggest one with a good amount of megapixels (about 8mp and up), with auto focus, and no aperture or shutterspeed options.
To ensure that your photograph represents your item correctly, avoid increasing the contrast too much or altering the colours. However a little cropping or straighting can improve your photograph.
I hope this helps! :)