Achievements are definitely the best place to start! Selling trinkets from your inventory is a good way to get pebbles starting out. The long game is getting a collection of pebble generating companions going on lots of wolves. Grab every Brother Perth and Brother Tavon you see in the Mole Market. They will pay themselves back over time and you can play with every 4 hours. During Seasonals, get as many Purple Background companions as you can. You can bond with them every 6 hours and they give you the most pebbles per play.
Also be sure to check out the Giveaway and Event forums for extra help starting out! There are lots of us that will happily give you resources and level 10 wolves!