I suggest you go to the library or go to you local Barnes&Nobel bookstore and buy used a text book on microbiology or better yet one of the excellent books on pathogenic microbiology.
You sound like a young student and perhaps many textbooks may be hard for you to read and understand. However, there are many groups of bacteria that are most useful to mankind. Think agriculture and think about animals, dairy, vegetation on your average farm. One clue I'll give to you, think of biochemistry.
Good luck. I'd avoid Google as it's full of misinformation. Go to a bookstore and buy a used textbook.
Best of luck.