what u do when u have to do keyword research for a particular site.Exact match or broad match???

It depends on the purpose of your Keyword Research. Like SEO or PPC.

If you are going for Organic SEO then you can consider Broad match search volume. When you work with a particular keyword in Organic SEO you get the benefit of its variations too.

In case you are trying to calculate projected ROI you can then take Exact Search Volume in consideration to be in safe side.

So in Organic SEO, create strategy and work considering the Broad match search volume and while Projected ROI calculation take the exact match search volume of those keywords.