It is exceedingly easy to come up with an original idea.
It's much harder to come up with an original idea that is true and correct. It's much much much harder to come up with an original idea that is true and correct as well as generally useful or insightful or explanatory.
The way you come up with good ideas is by coming up with lots of ideas and working really hard to honestly understand them, to prove or disprove them. After you've proven yourself wrong a couple hundred times, you'll hopefully start to see some patterns or some reasons or some principles for why you're wrong when you're wrong. In order to know how to be right, you must deeply understand how to be wrong. Finding a good idea is less about finding a good idea as it is about efficiently eliminating all the bad ones.
There's no straight path to good new ideas. You must go through this trial by fire. The master has failed more times than the beginner has even tried. The master is a master not because of his successes but because of his failures.