By listening to the conversation of others, you can get ideas for a scene or a character that will then carry your narrative.
2. Bring your characters to life!
Once you know who you imaginary characters are, pretend they're real and with you every day. Think about what they would be doing and what they might want to do. Get to know them, so that you can write them better than you could write yourself.
3. Muttering to yourself...just try it.
You need dialogue to sound natural, so try it out loud. It's ok if everyone around thinks you're a bit nuts. You'll be ok!