On the first day, or the day before if you don't want to waste a minute going downhill, you'll need to hire your mountain bike.
Mountain bike hire:
If you're in Bourg Saint Maurice, you can go to Gravitylab, it's a bike shop that offers sales and hire as well as all the equipment to protect you during your outing. Skiset if you're based in Arc 1600 or Arc 1800 and Evolution2 if you're in Arc 1950.
You'll also have the choice of taking mountain biking lessons at a school or trying it out on your own on the dedicated beginners' trails.
Once you've had some advice and hired your bike, you can start learning. You can start pedalling at the bottom of the resort to get to know your new companion for the next few days.
You'll then need to pick up your PASS VTT for half a day, a day or even a week before you can finally hit the slopes.