Although it is very common to drink coffee in the morning, definitely green tea is the national beverage in Japan. Green tea is drunk in Europe too but there is a one big difference between Japanese green tea and European green tea. The difference is that Japanese never put anything to the green tea. No sugar, no flavor. Japanese seeing a tea bag of green tea with lemon will be a same surprise as Italian seeing sashimi on the pizza.
Green tea can be bought in two types. The first type is classical loosen leaf tea type and the second one is more practical tea bag type. Green tea bags can be seen at self service drink bars and hotel rooms. Supermarkets sell more tea bags than the loosen leaf teas. Therefore if you want to buy the loosen leaf tea, better to buy at green tea shops. Also be aware that Japanese do not think English tea and green tea are the same category. Therefore you will never find green tea at shops which sell English teas.
Then there is a certain rules or customs to drink green tea. When you visit a company in Japan, almost no one asks you what to drink but automatically offers you the green tea. The green tea can be the one from the tea bag but you will never see the tea bag due to green tea will be prepared out of your sight.