Do you mean using it as self-treatment or using it to treat others? It's a lovely gentle therapy but, like all therapies, you do have to know what you're doing because some people can react badly to certain oils.
If you want to use it to treat others, you really need to take a training course. But there are a lot of good books on the subject, plus information online, if you just want to treat yourself.
However, you need to remember that aromatherapy has its limitations and works best in certain fields. It seems to be very good for conditions which might be termed psychosomatic - such as stress related illnesses, emotional problems and insomnia.