Standard CSV format is quote-separated with the fields in no particular order, similar to as follows:
"email@site.com","First","Last"
"email2@site.com","First","Last"
When you choose the file to import you should be presented with a "Field match" page where you can match up fields to data based on the first line of the file. ie:
email@site.com -> Email Address
First -> First Name
Last -> Last Name
Setting the fields this way should allow all data to be imported as expected.