Wow...this could be an entire study in an of itself. There are a ton of resources and many essential oil companies. Oils require study and preparation to use responsibly.

First, reference books/oils. I use Young Living oils but not exclusively. There is no need to join a multi-level to use essential oils.

One of my main reference books is by Dr. Scott A. Johnson: "Evidence Based Essential Oil Therapy: The Ultimate Guide to the Therapeutic and Clinical Application of Essential Oil." He is big on safety and this is a very well researched guide.

I also like Connie and Alan Higby's "Reference Guide for Essntial Oils". Very practical.

Robert Tisserand is a well known French aromatherapist and he wrote many books. The most well known is "Essential Oil Safety" and also "The Art of Aromatherapy" He consults with a company called Plant Therapy which is a very good reasonable source of EO's.

Also Dr. Kurt Schnaubelt, Ph.D (in aromatherapy) has written many great books. He owns a company called Original Swiss Aromatics which is an excellent company.

I have also bought oils from Healthy Traditions which is a great source for many things, not just essential oils. I also use NOW essential oils on occasion because they are a very reputable company.
There are many, many good essential oil books to choose from to get started. Some of these books are expensive but it is an investment in the proper knowledge to be able to treat successfully with EO's. There are many other essential oil companies you can buy oils from that are reputable. Read reviews and the information about where the oils are sourced from. For dosage? How often? It's not that cut and dry. It is a lifelong study, discipline and learning experience. Explore and read. I figure my investment has saved money in doctor/hospital bills. Anyway do the research...the rewards are great.