Afraid of Speech Delay

My baby aged 18M. Have yet say any word instead of mamma..ha..ha and humming and nod her head whenever i speak to her. But she understand well when i gave her instructions. Is there anyway i can make her speak? ? Because recent check up the doctor did mention that if by 2yo she still can say any word we need to go for speech delay therapy. We have 6 months left to get her to speak.