Locate information on Grandparents

My grandparents Frank and Julia Kohanski along with their childern Jan aka John, Antoinett and two others lived in what is today Kaunas Lithuania. Im trying to find out how I can get any information from that town. When they lived there the town name was Kowno or Kaunas. The came over to US in about 1906. My grandfather Frank was born in 1861, jan or john born 4/15/1903. Any information on how to get started would be very helpful.  Thanks   Tom