2007 Honda Civic spark plugs

I am at 51k miles on my civic, mostly highway miles, and I don't know when I should change the spark plugs. This is not something I would do myself, so I would like to know if it is something costly or relatively inexpensive? A few weeks ago when I turned the key in the ignition, there was a slow response. The engine turned over, but it took a while to catch. It seemed fine after that. But then it happened a couple of more times. I don't know what kind of problem this is, but I do want to take care of it before it turns into a bigger issue! Please help!