B757-200
Flight Crew Air Transport Pilot (ATP)
Confusion
Deviation / Discrepancy - Procedural Clearance; Ground Incursion Taxiway
Flight Crew Returned To Clearance
Airport; Human Factors
Cleared to taxi from point 'B' via taxiways 'B' and 'F' to holding point for 08R.  Miss-interpreted taxi diagram and turned south on 'N' taxiway.  Advised Ground Control and cleared to turn north on 'N' and rejoin 'B'.  There are currently no visible airport signs that define these taxiways.  Proper signage and more attention to taxi diagram.
B757 Captain reports being cleared to taxi via Taxiway B and F but inadvertently turns early onto Taxiway N at LPFG.  Taxiway signage and not consulting the airport diagram are cited as factors.