Who decides when and who gets to go into space

NASA has missions that are sent into space all the time. When the plans go into place, who is the one in charge to make that decision? Is it a group of members that makes the decision, the president? Also, does the government supply the money for the mission and if they do then what pocket does the government go to get it? Another question, how long would one have to prepare here on earth if a person were to go to Mars? Typically speaking, how many people are able to fit on a space shuttle?

Thanks for your help folks.

In the book, "Riding Rockets: The Outrageous Tales of a Space Shuttle Astronaut", retired astronaut Richard "Mike" Mullane talks a lot about that. It seems like one highly placed manager at the Johnson Space Center makes the final choice all by himself. This may have changed since the book was written though. The book was very critical of the process.

The Government...