Here's a guideline: start with const. If you later need to reassign the variable, change it to let. Most variables don't need reassignment.
const tax = 0.08 // Rate never changes
let total = 0 // Will accumulate values
const items = [] // Array itself is constant
items.push("apple") // But contents can change
Note that const prevents reassignment, not mutation. You can still modify object properties and array contents.