Depends on what kind of business u have, its location and type of customer base.
Post more details. But yelp is kinda expensive.

As bairaghi said, depends on kind of business, moreover type of customers, are they from 15-45 year range, do they use cell phone etc, if your business is service sector, yelp will be fruitful, I am in service sector and yelp has been best for me
Yelp is free to use,if you provide best service and if your clients gave you good reviews, then there is no cost, advertising in yelp costs money and I have not used it and its not worthy if you have good service and 5 star rating , good luck

It is for a service based small business, where we fix electronics for customers. I heard many good things about them, and the fact that advertising through them can boost my business by sending me more customers. Anyway, how expensive is it?

And also, since I am in a starting phase, I have no yelp reviews and sometimes when I ask people they don't have yelp to leave me feedback. So, what do you do to get feedbacks?

I think yelp is very useful for business. I myself use yelp a lot to find services and their reviews whenever I need something.