My suggestions are:
- don't be too interested in niche, it can be distracting
,
- avoid EMD domains, Google hates them (from my own experience),
- choose .com domain,
- don't build links in first 12 months,
- add content regularly, unique titles and descriptions with tags,
- find wider niche, and then splits this niche into categories:
domain.com
domain.com/category/smaller-niche-1
domain.com/category/smaller-niche-2
- be VERY patient.
For keyword research use Keyword planner and KWfinder.