Hi, Christine!
Not to worry! You are not alone in your newbie-ness, and we've all been there. (That's why we're here; to help!)
When you book Advanced Dining Reservations for a Walt Disney World restaurant, the credit card information that you are being asked to provide is simply to hold that reservation in your name. Should you either not show up for your reservation, or fail to cancel earlier than 24 hours prior to it, you will be charged $10 per person. When you DO show up for your ADR, your credit card will not be charged. Instead, your server will ask if you are on the Dining Plan. You'll be asked to provide your server with one person's Key to the World Card, which will have your DDP information printed on it. This will enable the server to guide you through the best use of your Dining Plan entitlements.
Regarding booking ADRs for Character Dining now for a trip in August? Not to worry there, either. Check out this list of
Character Dining options at Walt Disney World. I'm sure that you'll be able to find reservations for several of these great choices, especially because you're within about 120-150 days of check-in (Guests can only make reservations at 180 days of check-in at this time. It's not too late!) Just decide where you'd like to dine based on your itinerary each day and then book as soon as possible.
Keep sending questions as you think of them, Christine; we love to help!