Access vector elements with square brackets like arrays: numbers[0] gets the first element. Indexing starts at zero. You can also use at(): numbers.at(0) does the same thing. The difference: at() checks if the index is valid and throws an error if not.
Brackets don't check. I recommend at() while learning because it catches mistakes. Access numbers.at(10) on a 5-element vector and you get a clear error instead of undefined behavior.