Schmenke, its is good maintenance to re-index gears on any type of bike whether road, mountain or hybrid as the cables with stretch over time which will can result in gears slipping and shifters not functioning well. The amount you use your bike and number of miles you ride will affect the frequency you need to do this I.e re-indexing is done far more frequently on my road bike than my mountain bike (which does have thumb shifters). I would also try spraying WD40 as someone else said.
My other half does all if our bike maintenance although I did get a brief promotion to number one mechanic last year when he broke his wrist. He has built up a wealth of bike tools including a stand to help us do this. At a guess Schmeke, I would be inclined to say you should just take the bike to the shop, get it serviced and let them sort it although if it was me I'd try re-indexing first as this is a task you can do yourself which might fix the problem without the need to buy tools, spares or requiring you to take things apart or to a shop. :p Always try the simple fix first. :)