What should I look for in a digital controller or decoder? I am new to the hobby.

With Digital equipment, it is always a good idea to use one that is NMRA / European Model rail Standards compliant. This will make it much easier to run trains with any compatible Digital controller. The beauty of using a Digital system to control your trains is that you don't have to worry about complicated wiring - only 2 wires to the track in many cases. Here are some links to other sites which you may find helpful:

http://dmoz.org/Recreation/Models/Railroad/DCC/
http://www.railsusa.com/links/Model_Railroad_Electronics/
http://www.dccinfo.com/