You'll need to deposit some funds into your online trading account[1] before you can buy shares. Be sure to check the terms to find out what the minimum account balance is. Some online brokerages might let you trade with as little as $250, others might require $1000. And most will charge a fee if your account balance (cash + value of stocks) is below a certain value at the end of a quarter. E-Trade's threshhold is either $4000 or $5000. Some online brokerages might also charge an "inactivity" fee unless you make a certain number of trades per quarter. BTW, I didn't think you could buy securities with a credit card though you can buy some types of treasury bonds.
[1] I'd recommend staying away from margin accounts if you're just getting started. With a cash account, the most money you can lose is whatever you deposit in the account. With a margin account, the brokerage essentially gives you a loan. You can lose alot more than the funds you deposit in the account. And it's not just a matter of gritting your teeth and waiting for your stocks to rise. With a margin account, your brokerage can force you to sell (called a "margin call") if your margin stocks decline too far. Margin accounts do let you trade options and such but I'd start with a cash account...less risk.