Could you please help with explanation

I have been a professional website developer since 2009.
I've worked as freelancer developer and still working but made little money per month to take care of my bills.

Later in 2014, I learned from warriorforum that I could change my life as an affiliate marketer.
I bought books, read book, watched videos then I setup domain and hosting, designed websites, and put up contents.

I could not advertise them because of low income. But since I joined CPAHero, I have come to understand that there are things like:
- High competition niches and Low competition niches
- Also high competition keywords and low competition keywords.

Could you explain them please? Also stating in good details what you advise newbie like us to get into.
I'm having strong feeling they are the reasons why I have not made 1cent from affiliate marketing.

Regards,

Christogonus.