1. Use the validWhen validation rule.
This link explains how to use it.
This will work only if you're just validating that some value was placed in the credit card number field. If you need to do a check-sum or anything more sophisticated than that, you will have to write your own custom validation rule in order to make this work.
2. Code the "Select One" option like this:
<html:option value="">Select One</html:option>
If you do it this way, the required validation will work because the value of creditType will be a null
string if this option is selected.