What causes yellow discharge and how can I be...

causes and treatment of yellow discharges:
its normal for a vagina to have a slight discharge, but offensive and bad discharge is never OK
most of the times women report that odor as a fishy odor , it may indicates an infection to your vagina and should be identified and treated appropriately 
three main conditions can result in a vaginal discharge and bad odor 
Bacterial vaginosis — an overgrowth of normally occurring vaginal bacteria — is the most common vaginal infection that causes a vaginal odor. 
Trichomoniasis — a sexually transmitted infection — also can lead to vaginal odor. 
Chlamydia and gonorrhea infections usually don't cause vaginal odors. but can cause itching and redness and discharge 
treatment depends on the underlying cause 
most cases can be treated with metronidazol 500 tap three times daily with topical application of gynydactazol cream or suppository 
its important to treat the sexual partner as well 
diagnosis:
to diagnosis such condition  you should visit your gynecologist who may examine the area and sent some samples from the discharge to be examined
symptoms:
most of the cases can present with the following symptoms 
itch 
bad odor 
discharge 
redness
painful urination 
painful sex.