First Contact

The term "First Contact" describes the first official encounter between representatives of two races or governments. Occasionally, the Official First Contact takes place years or even decades after members of the species involved have first met.

Traditionally, first contact is only initiated by the Federation if a civilization is sufficiently advanced. Normally, the civilization must have developed a form of interstellar travel (Typically Warp Engines). Contact with more primitive civilizations is prohibited by the Prime Directive.

It has been stated that approaching scientists and intellectual leaders first, in private, is a preferred method of first contact, on the assumption that they would have minds open enough to easily grasp the concept of alien life.

Source: Memory Alpha