Whilst it is nearly impossible as a muggle to get into the Ministry of Magic, you can still visit the location in the films. This is found in Westminster, home to Muggle politicians.
Here you will find the exteriors used for the Ministry of Magic during filming for the Polyjuice potion scenes in Harry Potter and the Deathly Hallows part one.
Shots taken here are also used for the trip that Mr. Weasley and Harry Take in Harry Potter and the Order of the Phoenix.
In the books and the films, there is a red telephone box outside the Ministry of Magic that wizards use to access the Ministry.
Unfortunately, this telephone box was just a prop and never existed in real life.