Are all the planet slots in a galaxy vacant planets? If no how do you find a vacant planet? Is it just hit and miss? Is one section of the galaxy more likely than another to have empty planets?? Just a noob asking. Almost ready I think to start colonizing?

Just use the galaxy view. If the slot is not already occupied by a planet or a recently destroyed planet, you should be fine.

The only caveat there is that if someone else has sent it to the same exact slot before you, the ship will return.