Won't start battery/starter are good, HELP

My 2000 expedition will not start, the battery is new as of Oct. 09 and I had the starter pulled and tested and it is good. The radio will not come on unless it has sat untouched for a day but once you try to start the car it turns off and will not come back on. I am not sure what to do next. If anyone can help me I would truely appreciate it.
Thank you, Susie

Test the battery, sounds like it's probably dead. Did you try jump starting it?