Welcome to FT!

I haven't flown Southwest in a while, but you can almost certainly print your boarding pass at home, avoiding the check-in line at the airport (though if you're checking bags it won't make much difference as you'll still have to hand them to an agent).

Otherwise it's all pretty self-explanatory. Your biggest risk is probably getting yelled at by the TSA. Just read up on the annoying rules (liquids in containers of 3oz or less, all together in a separate 1 quart ziploc bag; shoes off, laptop out of bag, and so on...) and you'll be fine. A 55-minute connection in LAS should be no problem; if your incoming flight is late, they'll put you on a later connection to LAX.