We did this and it worked great. Got our pass online using the wifi in our apartment in Paris the night before we needed them. A U.S. credit card is fine, doesn't need a chip and pin, because you'll be entering all the card info on the online form. Once submitted, the system sends you an email with your code to unlock the bike. That's all you need., and this way you don't have to insert your card at the bike rack when checking a bike out.
Get the official Velib app for your phone. It's free, and provides a map of where all the bike racks are, how many bikes are available to be checked out at each one at any given moment, and how many open slots there are to check the bike back in.
If you get to a rack that's full, you can punch in your code at the kiosk, and the system will give you another 15 minutes to find the next nearest rack that has open slots.
Every bike comes with a lock, so if you need to duck into a store for something, you can lock it to a railing or bike rack. But the racks that houses the bikes are self-locking, so don't use the cable lock on those!
Finally, as another poster suggested, these are just for point-to-point travel. Take one from Ile de Cite to the Eiffel Tower (about a 15 minute ride) and then find a nearby rack to return the bike to. When you're ready to go to your next destination, just unlock a different bike from the nearest rack, and the free 30 minute clock is reset. We did that all over town. If you go over your allotted free time, it's just 1 euro for the next 30 minutes. But the price goes up steeply from there, to discourage folks from checking out a bike and keeping it for hours at a time.
Start using the app now to get a sense of the ebb and flow to how bikes and open spaces are available in different neighborhoods throughout the day. There's definitely a pattern which will help you determine the best time for your plans to use a bike to get to and from different areas of town.