I am currently a coach/player of a group of 14-16 year olds at my high school... We want to start a basketball team and most of us aren't too experienced. I want to know the top priority practice plans I should focus on with my players over the summer? Are there any important fundamentals I should prioritize given a time constraint?

Here's a list of the most important basketball skills to focus on:
https://oneupbasketball.com/most-important-basketball-skills/

In addition, teach them good defense, rebounding, ball security, and basic motion offense concepts. That will give you a good foundation.