which is best, focus on low traffic keywords or High

we lunched a new website, when we focus on SERP and keywords ; which is good ? low or High traffic keywords

Why not all of them?, low will rank sooner, medium will take longer and high can be ranked in time. Don't post 500 word articles based on 1 long tail keyword, instead write or have an article of 1500+ words written targeting lots of keywords but without keyword stuffing. Optimize META titles and descriptions, promote via social media, social signals, and build quality backlinks. New post, rinse and repeat. This is a basic outline but it should give you some ideas.

focus on the site, make it original and worth visiting, then you don't have to worry about keywords too much as they will depend on what your content is about