Generally:
If <30 and low suspicion (no nipple discharge, solitary mass, no lymphadenopathy, mobile, smooth borders, etc.) - you'd likely ultrasound and follow. Women under 30 have too dense of breast tissue for the mammo to give much worthwhile info, so if the u/s looks suspicious --> biopsy.
If >30, mammogram
Mammo normal-- low suspicion --> Ultrasound possibly followed by biopsy if u/s looks suspicious
Mammo normal-- high suspicion --> biopsy
Mammo abnormal --> biopsy