Deciding on which ideas to pursue

Read Hamming's famous talk on how to decide what to work on. He wrote it for research scientists but the same logic applies: http://www.cs.virginia.edu/~robins/YouAndYourResearch.html

(The talk is entitled "You and Your Research" so if this site I liked to gets overloaded, just search for that).

Finding the right mentor takes time but is worth it.

I agree with finding a mentor.