Two words: Social. Medial. Several words: Facebook, Twitter, Instagram, LinkedIn, Angie's List....
Word of Mouth is good but do NOT rely on it. You will be homeless unless you're already VERY well established.
And Yelp is fine, but stay away from their paid advertising. I got suckered into that crap. They wanted $350/MONTH!!! Worst advertising (in comparison to cost) EVER. I could post 7 $50 ads on Facebook per month and reach 10,000x's the potential clientel.
The only good thing about Yelp is that they are actually searching for you. Still, had it for 6 months and hated it. (btw, i was the ONLY detailer in my area to have ALL 5 star reviews. 10 reviews. They ghosted SEVEN of them. Why? I didn't know the people before detailing their vehicles. Not like it was friends posting. They did it because I would take all the business from the companies that pay Yelp and have 2, 3 and 4 star reviews.)