Take it back to the store. You'll just end up trying to fix the problem by yourself by throwing more money at it. Visit a few reputable bike shops and see who answers your questions honestly without any hesitation. Make sure you tell them what your budget is and pick up even an entry level bike that suits your purpose. 1 year free maintenance is usualy included, build a good relationship with that shop and come back in a year or so and get a better bike. They will be thankful for the business and you will enjoy not being a full-time mechanic with the dept store bike. Good Luck and Ride Hard!