My son is 11 years old and getting started.
Here are some things to keep in mind...
Start with bodyweight movements.
Work on form and progression, form being the priority at first.
Stay with bodyweight movements until a strength base is built.
Progress as below...
Bodyweight squats. When you can do 50 bodyweight squats then move on to goblet squats.
Push-ups. When you can do 50 push-ups then move on to bar dips and eventually bench press.
Chin-ups. When you can do 10 chin-ups then you can move onto barbell rows.
Start off correctly and include sit-ups and conditioning.
So to start; Use body squats, push-ups, chin-ups, sit-ups and conditioning.