Lenspen or clean microfibre cloth + isopropyl alcohol (that's what most optical cleaning fluids are) for stubborn marks if necessary. Most modern lens surfaces are pretty tough but use a blower to make sure there's no dust or grit on the lens before you use anything else, or there's a risk of scratching the glass.
Don't overdue it. There's usually no need to clean lenses unless they're very grubby.