There are a lot of great resources available for coaches. Don't feel like you have to start from scratch. The resources below are great places to learn about planning practices, running drills, and helping kids improve.

Little League International Resources

Tee-Ball Resources: https://www.littleleague.org/play-little-league/tee-ball/
Coach-Pitch Resources: https://www.littleleague.org/play-little-league/coach-pitch/
These links provide everything from individual drills to detailed instructions for a full season of practices for tee-ball and Rookies coaches. You can customize these plans as you see fit or use them as-is.

Little League University: https://www.littleleague.org/university/coaches/
Little League University provides a wealth of information, ranging from how to operate a league to how to coach a team. This link directs you to  resources related to coaches. There are videos of drills, explanations of rules, and a variety of other pages useful to coaches at all levels.