What you are asking about is keyword research. There are many ways you can find new potentially profitable kw, but you will have to 'research" it.
Use Google's Keyword Planner, farm Google's "People also asked", and Google's "Autocomplete". There are a few other avenues to include for more suggestions to consider. If you want to proceed at a faster rate than doing all this manually, buy a tool take collects and correlates the data for you. (Personally I use Rank Tracker).
Do not confuse Keyword Difficulty, with KEI, (keyword effectiveness index). KD is an Adwords term and sometimes it correlates to finding easy low competition keywords. KEI on the other hand is quite effective in finding potential new kw's.
The simplified formula for KEI is :
KEI = Searches squared / number of pages competing for the keyword
So you can use Google's Keyword Planner to find the number of searches and then you just search Google and the SERPS will provide the number of competing pages...(google gives you that info in the top left of the pages, generally displayed as " about 4,100,000 pages in .39 seconds"). Now all you have to do is the math.
( This is where a good paid tool excels !!!)