Plain and simple...
Make a short list of 10-15 keywords/keyword phrases you can think of for your niche.
Example: Dog training niche - keywords: dog training, obedience training, pet training, etc.
Got to google in your browser. Do a search for ' keyword suggestion tool '.
Select the ' adwords suggestion tool ' link ending in /sktool.
Enter your list of keywords into the tool, one at a time and you can either save the cvs or excel database file in the bottom right hand corner of the results page, or you can write out the best keywords for your niche from those results on a pad, based on their unique search traffic volume results for the month.
I do both, selecting a top list of 20 keywords to write down and promote as my first keyword targets to promote around and build backlinks to my site with.
That will get you started on the right path. Repeat the process over and over when you want to expand your listings and rank and it will take on a life of its own, becoming second nature to you to create seo and content around that list each and everytime you need it within minutes.
Hope this helps.