ilovemyyoni has given you some good info. A whitish or clear, virtually unscented discharge is normal and healthy. A yellow or yellowish discharge may indicate an infection. Yeast or thrush will cause redness on the vulva, irritation - more of a burning than itch, and a white often clumpy or cottage cheese-ish discharge. That is treated with monostat. Plain yogurt, both eaten and used inter vaginally will help too as will taking acidophilous and eliminating sugar and refined flour porducts.

The yellow discharge sounds more like a bacterial infection. That can be caused by a number of things. Most groceries, Target, Walmart, drug stores, have a section of feminine care products and you can find something there to treat this, it will cost $8 to $16. However in this age of STIs its a good idea to get tested if there is any possibility you could have gotten one. Some produce few or only minor symptoms but left untreated can cause problems such as sterility, inflamation and pain. Most places in the US (I don't know where you are) have a county health service where you can get tested, often free and without parental consent. The testing is easy, they pretty much just rub a qtip in your vagina.

Avoiding clothing that is tight in the crotch, thongs (which act as a wick and move bacteria from the anal area to the vagina) panty hose and wearing cotton undies and no undies at night can help avoid these infections. Also use only unscented, non-deodorant soap or no soap when washing your vulva area, do not use scented or deodorant pads, tampons or TP, and no perfummy stuff down there.