How can I find the right keywords for my niche?

You can use the Google keyword planner to find out keywords related to your niche -

https://adwords.google.com/home/tools/keyword-planner/

Select "Search for new keywords using phrase website or category"

http://prntscr.com/fw4fyb

Enter your main keywords in the planner and generate ideas.

You can also enter a competitor website URL in the box which says "Your landing page" to see what keywords they target.

Once the planner gives you ideas, you will be able to find out many relevant keywords that you can target.