yep, your cables have stretched since new...its normal.
either adjust the cable or drop it back to the shop, they usually cover the first service (because it takes seconds) for free.
if you fancy it your self its pretty straightforward.
get someone to lift the back of the bike, drop the gears to the lowest sprocket on the block (the rear cog thing) and then click one gear up....turning the pedals all the time. if the gear doesn't shift up (which it most likely wont as the cable has stretched) then turn the barrel adjuster (small plastic tumbler) a tiny bit until the chain pops up onto the next sprocket.
then turn the same barrel back a little to let it sit smoothly.
if you do this to the one gear all the others will follow suit.
here's a neat video to walk you through it. this fella chats for a while about the stop limit screws...which you don't need to adjust...yet.