you go to the store and hand over your money. which one depends on your computer...
For Simcity 4 Deluxe, i really, strongly, urge and recommend at least a 2.0 gigahertz processor with one gigabyte of ram (although i've played it pretty well with 512 on my dad's computer, but not maxed out graphics.) a 128 megabyte videocard is also good...and you'll need to be willing to shut off background processes that consume a lot of RAM while you play (CTRL+ALT+DELETE brings up Task Manager; a lot of virus scans eat up a lot of resources, so i recommend shutting them off and unplugging the internet while you play.)
Simcity 4 Deluxe can be challenging, and with each Sim City, it took me at least one city/region to play and trash to understand how to play the game well. once you play the game, go to Simtropolis.com, register, read the FAQs and start downloading custom buildings.
Simcity 3 is cheap ($5-10 at Best Buy,) can be played on most any computer that has been purchased in the past 5 years, and is an easier game...although the graphics aren't as good.
"The vapors! The fainting couch! Those heartless elitists are burning down the plantation with their logic and arithmetic!"