How do you get into special forces?

You have to join the military first, then if you show a high aptitude and an interest in joining the special forces you may get given the oppurtunity to try for selection.
However, the standard is very high during selection, training and in service.
Also, you must make sure that you join the correct branch of the military, ie;(assuming you're either American or British) if you join the Royal Navy/ Royal Marines you could be selected for the SBS, US Navy/ US Marines you could be selected for the Navy SEALS. But in either the British or US Armies you could go for the SAS or DELTA force, respectively.