Since 45 minutes the american version of civilization 5 and the demo for the rest of the world can be played.
The demo contains 3 different civilizations on a small continents map with standard speed. The demo can be played for 100 rounds, all technologies until the middle ages are available.
Size of the demo is between 1 and 4 GB, depending on what you've downloaded before and can be found under steam://install/65900 on steam. You need to have a steam account to play the demo.
Steam is a online distribution service. You don't have to pay for an account there, but you have to create one. Please read the subscriber agreement before you create an account.
For general civilization 5 questions please look into this forum, for technical problems with e.g. steam please into this forum. Please also visit all other forums on civfanatics and have fun .