If the wood finish is already sealed and in good shape, there isn't much to do to it but put some paste wax on it. But if the finish is an oil finish, like a few of my rifles, I just add a little boiled linseed oil from time to time. Put a few drops in the palm of one hand, rub hands together and then slowly rub it on the stock. It'll look great when the oil added. And that will keep rain out. If you use a true varnish, which I don't recommend on a stock that's in good shape already, it'll look good but will be very hard to repair if you put a deep scratch in the varnish. That said, I have used the Minwax Fast Drying Polyurethane on some cheaper 22 rifle stocks. You can buy it in spray cans. Goes on easy, flattens nicely and dries very fast. Just don't overspray and cause the varnish to run or drip. My usual approach with the spray varnish is to take the stock buttplate off, which I'm sure you'll do in any case, put a small screw eyelet in the butt and hang the stock and then spray it. You need good lighting to do that well.
Try the paste wax first. Cheap and easy and you can't possibly mess up the stock. And...if it's an oil finish, rub a little linseed oil on it like I said. That also can't mess up a stock.