What kind of computer do I use to program an atmega?
To compile programs for the ATmega you need a so called toolchain. A well used one is the GNU AVR toolchain (and it is  FLOSS). Linux (best), Mac and Windows can handle it. To get the compiled code into your microcontroller, you need a programmer like AVRdude. The corresponding hardware is called an In-System-Programmer (ISP). There are a lot of projects out there, like the USBtinyISP. Atmel has an application note on in system programming (AVR910 pdf) explaining a lot of things.