Take a 25 minute walk through of the iDempiere Freedom ERP Project, demonstrating how it works, and where to find resources and the community around it. Then make your choice if you want to participate in this project. Note that it is free not as free lunch but freedom to modify and share code. So either you are willing to learn and share or pay competitively for it. This ERP is for very complex challenges at SAP level and I advice you to measure against your actual needs in case a lesser ERP can handle it.
After you decided to go ahead you can take a deeper plunge here http://www.youtube.com/playlist?list=PL2805F9B642C0C91C
AND read up PDF guides here http://sourceforge.net/projects/red1/files/ADempiere%20PDFs/