Assuming you're talking about an African voilet (Saintpaulia ionantha), we often purchase them planted in poor soil to begin with. They like 2 parts good rich pasteurized soil + 1 part peat, which you can purchase in a bag like any house plant soil.
It should have good drainage, as the "feet" cannot tolerate staying wet for long. They like to get a tad on the dry side before the next watering. It's VERY important that you do not get the leaves wet, they will rot.
There are many great resources that you will find helpful. Try the internet or library. Hrere's a source that may help: http://www.uri.edu/ce/factsheets/sheets/africanvio...
This site lists different diseases and pest problems your violet may be experiencing, as well as general care.