In terms of clothing, you probably already have a breathable technical T-shirt, shorts and a windbreaker? That's all you'll need to start trail running.
In terms of accessories, you'll need to wear class 3 sunglasses (but most of them are, except for the fashionable ones), something to keep you hydrated (a belt with a flask is best, but a small water bottle in your hand is also good for a start), a watch with a GPS so you don't get lost, or your smartphone with the GPS map you downloaded before you set off, as there isn't always much of a network in the mountains.
What about poles? You won't need them for your first trail; they're really more essential for routes with steep positive and negative gradients. They will certainly be a major asset if you decide to continue this sport, but they will be cumbersome for a first outing.
What trail can I do in Les Arcs?
There are several, which you can find on the Les Arcs Tourist Office website, but for a first run, Le Petit Balcon Nord seems ideal, with 9.68km and 209m of ascent.
It's very accessible and offers beautiful panoramic views over the villages of the Versant du Soleil. It's a beautiful loop starting from the funicular railway, quite hilly over the first 5km and with much less ascent on the second part. For a first trail, you'll need to allow at least two hours, as you'll need to take a few breaks to take in the scenery.
GPX route for this trail.
And if you liked the first one, you can follow it up with a second trail, the Les Arcs Trail Funiculaire. It starts the same way and finishes at Les Arcs 1600, with 810m of ascent over a distance of 5km. Your thighs will remember it.
GPX route for this trail.