if you have a decent soundcard with stereo outputs then connect them using a decent phono cable such as this
If you don't have a decent soundcard then your option is to use your computers 3.5mm output, you could use one of these
If you don't have a decent soundcard then to get the most out of these speakers i advise you to consider spending a few ££ on a decent external soundcard that will plug into a USB socket and take control of all sound processing duties, a cheap option is something like this , a slightly dearer option is something like this.
But if it was my cash i was spending i'd seriously consider this Creative Soundblaster , It'd be a pretty decent match for the Rokit's without busting the bank - this is a surprisingly capable soundcard, i bought one for my brothers laptop speaker system. Connect this to your computer using a usb lead or the optical input then connect the soundcard to the Rokits using a bog standard stereo phono lead as above. You can pick these soundcards up for £60ish, budget another £10 for cabling and you will have a pretty awesome sounding system.