Your best bet is to go to Google planner if you haven 't got an account then you can sign up for one for free, then add your seed keyword like, "dog training", then hit 'get ideas', on the next page click on the keyword idea's. There you have all your keywords and you can use the filters on the left side if you like to dig down more, for eg. you can filter keywords with more or less than say a 1000, and you can filter the CPC (cost per click) this is mainly used for people paying for ads.
You can also use the include exclude option for sorting out words like, free, sale, what ever you like.
Hope that helps you a bit.