This is my opinion, Correct me if I am wrong.
Q1) How do you get the niche idea?
You might be making a mistake if you choose the micro niche based on what topics you like, as it may happen that you have chosen a micro niche for e.g. Golf then you might not get High CPC or High volume keywords or you might not get a proper affiliate product for that. So what you have to do while choosing a great micro niche is do a good research about the hot niches that are running in the market right now. For that you can choose Google Trends, Buzzfeed or even social media to get that. Choose the mirco niche which gives you high CPC or good affiliate commissions.
Q2) After getting the idea of how do you search for keywords?
After you get the Idea and your main keyword, find long tail keywords. There are several ways to get the keyword ideas: 1) Google Keyword Planner, 2) Ubersuggest by Neil Patel, 3) Google auto Suggest.
1) Google keyword planner: Go to G planner and search your main keyword there. Make sure you select proper filters like nation and competition. You will get a huge list of the keywords. From that select the Low competition keywords with an average search volume (at least 1k a month).
2) Ubersuggest: Like G Planner, Search your Keyword there. You will get a list of keywords there too. From that choose those keywords who have high CPC and low competition.
3) Google auto suggest: Open Google, paste your keyword in the search and wait. You will get the suggestions below the search bar completing the keyword. You can also find long tail keywords at the end of the Search Result Page.
And don't rely on the keywords on the basis of competition. They usually shows you the competition of PPC. So check all the keywords manually in Google under double quotes. Select the keywords based on the number of search results that you see.
Note: Only choose those long tails keywords who have at least 4-5 phrases in it.
Q3) How do you validate your idea (whether it's worth your time and efforts or not?)
After getting the ideas and keywords, Write in depth and great quality contents (If you want to know what I am talking about checkout the contents of greatist.com). If you cannot write good then hire a writer who can do for you. Write high quality and lengthy contents (1500+ words).
I hope it helps.
Thank you!