Hmm, that's a good question. Depends on the situation as idea in itself can be just small one or require good amount of planning with combination of other ideas. As for one example, one friend once told that he had been wondering for way to solve very complicated technical problem for a longer time already and one night when he was awake at bed in the middle of night he got idea how things could be done. In a way, idea could be described as a result of wondering about things and a basis for actual actionable plan to do something real. Different thing is if the plan is actionable in the real life but the mere combination of thoughts could be called as an idea.
As for if the idea needs to be "new", it certainly does not need to be. It's just that world is full of ideas but not many of them will survive to the surface since for most of ideas there aren't anyone willing to use their time to actually do it. Also, as said more than few thousand years ago, there is nothing new under the sun. Most of the ideas are combination of previous knowledge or other ideas and while the amount of knowledge increases, the number of thrown away ideas does so too.