With AutoCAD there are currently two languages you can use for programing/customization for the power user, these include AutoLisp (Visual Lisp) and VBA (Visual Basic). Below are three resources that you may find useful along your programming path.
1. One good stop for VB is the Expresso Code Cafe. Here you will find quite a bit of good information related to the AutoCAD object model and tons of sample code to help get you started. Be sure to check out the "Brain Shots" forum. Related to this site is CAD En Coding - Information for the CAD Programer.
2. AfraLISP - this site belongs to Kenny Ramage and has a wealth of information for lisp and VBA.
3. AcadX/ - this site has quite a bit of code and articles. Be sure to check out each link in the applet on the left.
Happy Coding...