Waiting til Junior year is ok since a lot of places places don’t let you shadow unless you are 16 or 18. You can do a few things before like pet sit for a neighbor, volunteer to help an elderly neighbor or friend or younger children. Any hours you can get with those early are good.
Junior colleges are great for math, english, humanities to get some of those out of the way.
Example Freshman year hs my son spent the summer as a senior camper, helped with horses and only extracurricular were football and golf. Sophomore Jr Councellor at camp for summer, assist in horseback, played polo during school year nothing else. Junior year worked part time at office supply store, played polo, horseback instructor/councellor all summer. Sr. year played polo, got certified as a vet assistant, worked as vet assistant, volunteered with a hospice during school year. In summer travelled to England to visit a friend, worked with a travelling equine vet all summer.
You can see the majority of his experience started later.