It depends on what your classfied adverts are. For example if they are car adverts, write about the latest cars and write reviews etc. Look at what your competitors are doing right and use that as a basis for your ideas.
I can see that would work if you have a specialised site, but the OP just says “classifieds”, which makes me assume that it’s general-purpose, covering anything and everything. Much harder to see how a blog would fit in that scenario.
(I’m also aware that the OP has not returned to this topic since the day he created it, so seems to have lost interest.)
Along with On-Page Optimization Backlinks with proper anchor text are very important to get top rankings.