I am in doubt if I should share new academic opportunities, studies, etc with college's friends, because they will may be my future competition for a job and they may get the job thanks to the help I gave to them. What do you guys think about this. Am I being selfish?
Yes - it's not only good practice to share new information about career opportunities or research findings with your friends, but that's one way to build a healthy network of collaborators and a healthy community. Even if there's a chance that a friend gets a job you were hoping for, ideally they remain your friend and you can learn from their experience second-hand.
It's also good practice to share new discoveries with people who aren't explicitly your friends, both from the perspective of open science and because, well, you're probably not going to apply for every job you see or write every paper you can conceive.