My 3 year old son doesnt talk. Why?

My son only says maybe about 10 words, but not clear. He only says a word at a time not together. Sometimes when he trys to talk it sounds like he is huming the word. I know he can hear. He lets me know when he hears a firetruck or police car go by. He does what i ask him to do. Like if i as him to bring me one of his toys he does. If i say bring me your car he does or if i say bring elmo to me he does that to or if i ask him to throw something away he does. When he wants something to drink like milk or juice he will get it and bring it to me. So i was just wondering what was wrong. I am taking him to the doctor. What will the doctor do and what happens in speach cles?