What to write down
Date, organisation, hours, and a sentence on the work. The sentence is the part people skip and the part that turns out to matter — "ran the Saturday intake shift" is usable two years later in a way that "volunteering, 4 hours" is not.
A supervisor's name and an email address for anything that might need signing off. Organisations lose staff, and the person who can verify your hours in ninth grade may not be there in twelfth.
If the work changed over time — you started sorting donations and ended up training new volunteers — record that when it changes. Progression is the most interesting thing about a long commitment and it is invisible in an hours total.