I was nearly 30 years older than you when I discovered metals, so I applaud you for starting young.
I'd leave that $600 in the savings for any emergencies that may pop up. It will be pretty difficult to trade a couple ASE's for a new Diehard at Sears if the need arises.
Probably too young to get a credit card, but if you can get one here's what I would do.
Obtain a card with a smallish limit, say $400. You can use it to buy a 5oz Silvertowne bar, then pay off the card bill every month.
That way you start building a good credit record and get some silver too. Just have to be disciplined with a card.
The Silvertowne bars are generic, but they take credit cards and offer free shipping. Cost is slightly higher, but you'll be paying $6-10 shipping on small orders anyway. Also be aware that most online dealers charge an extra 3% for using a credit card, which can really add up over the long run.
Could also buy just a couple ounces of silver one month, then have a little extra to buy some low premium gold the next. European 20 franc coins can be had for about $325 each right now.
Just an opinion, so do some research, listen to ALL opinions and make your own decisions.
Good luck!