Enterprise (TNG) makes first contact with a Tamarian ship at the planet El-Adrel, in the 'Darmok' episode. The Tamarians had been transmitting signals toward Federation space for weeks so were probably aware of 'alien intelligence' for at least that length of time. There is no mention of them being unaware of aliens or a part of an aliance prior to this encounter.
Their ship though, was roughly equal in armament and technology to the Enterprise. Maybe even a bit tougher. Tamarian captain Dathon and Captain Picard become friends during the encounter and though Dathan is ultimately killed, both sides see this first contact as a good start. Dathan records in his log, "Picard and Dathon at El-Adrel", which Picard gives this to Dathan's First Officer who then withdraws back to Tamarian space. It does seem that is was actually 'they' who initiated first contact or at least invited it. A tentative first attempt at contacting aliens?
Per Memory Alpha: "Picard and Dathon at El-Adrel" means, a "Successful first contact between two alien cultures, or to work toward a common goal", in the Tamarian language.