I'm betting that the cable is too slack. If it worked okay when you first bought it, then the cable has either slipped or has stretched just a little... that's normal. The cable is easy to adjust... once you've seen it done.
I'm going to suggest that you take it to the shop, and ask if you can watch as the wrench fixes it. You'll be good to go in no time.
Two unsolicited tips:
Buy a set of hex keys. Figure out which ones fit the derailler cable pinch bolts, the brake, stem, and seat bolts, and take them with you when you ride. Stuff works loose, and a cheapo hex key can keep stuff from failing... or falling off.
If a shifter suddenly gets weird... It hangs up between gears, is extremely sluggish, or all of a sudden doesn't want to index on the right gear... or maybe indexes between gears... You have a cable that is just about ready to break. Typically, it'll act weird for a couple of rides before it snaps, so you'll have ample warning. If you even suspect a failing cable, remove and inspect it as soon as you can... It'll be frayed, with broken strands, at either the shifter end or the derailler pinch bolt. Don't ignore it. If it breaks out on the road or trail, you'll have to McGuyver a repair that'll get you home.