Prioritization is key
As a (new) manager, one of your most important activities will be to prioritize work to be done by your team.
Get into the habit of thinking not only if some given task/project is worthwhile, but also how worthwhile.
You will never have infinite resources or people in your team, so some tasks or projects must by necessity be dropped to the benefit of others.
Value/Effort ratio instead of just Value
Every task or project has an expected Value–the benefit (financial or otherwise) it brings to the team or organization.
But the Value alone is not what you should use to prioritize tasks. A better metric to use is Value/Effort, whereby you also take into account the Effort (i.e. man-hours) needed to accomplish the task.
There are many other dimensions that must also be addressed in order to prioritize, naturally (emergency tasks, unblocking tasks).