The only way to ensure you will be A boarding in advance is to pay for Business Select. However, you can almost always pay $40 dollars at the gate to get BS boarding number.
I hope you realize that there are no "1st class" seats. The only thing that A boarding gets you is earlier boarding and a selection of whichever coach seats you want that have not previously been sat in. Southwest is open seating. The next person on the plane takes the next seat that they want.