Overweight and looking where to start on stair climber and treadmill

I just started a job at a new company and they have a very small bare bones gym in the building. Today I brought clothes with me to go to the gym for the first time! :)

When I scoped out the gym, I saw two things I was interested in - a stair climber and a treadmill.

What should I aim to do on each of these for a first day/week? For context I’m very out of shape - female, 5 feet 3 inches, 240 lbs, but love to snowboard and want to get more in shape for snowboarding season!