if i recall right from my research, you just can't. when i was 19 i would have my sister rent me cars and i'd drive them; we'd just take the risk. if you get pulled over its not prison or anything, as long as you have a license. although you are going against the contract terms of the car rental place and they could sue you, but they'd never know unless you crashed it or something.
we risked it, it worked.
wait till you're 21.
oh, when i was 19 i also rented a uhaul truck once cause i needed "vehicular transport" so badly. you can do it without problems if you're over 18 and it doesn't cost extra. but they charge per mile.