Poloniex is an exchange that helps you buy and sell crypto currencies and using fiat is impossible so you should use an exchange to buy using your credit card and then send the funds to your poloniex wallet which I don't recommend by the way.
You probably don't have much but Its never suggested to do that , online exchange don't give you control over your private keys so If It happens and the exchange goes down (hack , ddos .. anything) , you won't be able to access your funds anymore.
You didn't mention the country you live in so I will suggest buying from virwox.com as they don't ask for ID verification's or anything but the fees are definitely high.
I'm a complete beginner, I'm reading my 2nd book on cryptos but I don't know even the basics of purchasing and securely storing. I am in the process of opening a poloniex account, I was going to use coinbase but a) they couldn't even do a debit transaction for me, and b) they've been hacked once or twice already. So I thought I'd try poloniex, then keep a local wallet on my desktop and backed up on a couple of usb memory sticks. You sound like you know how things work well enough to give me some good advice.
If you have the time I would appreciate very much if you would tell me what to study up on. This page includes advise to use Tether and Electrum. Do you agree with that?
I live in the US and would use a debit card or bank wire xfer to depost funds somewhere that I can buy Bitcoin, Ethereum, and Litecoin for starters. I have a few more I'm researching but I want to buy before prices rocket up again.
TIA,
Mike