You'll need a Y connection, headphone to phonos.. sometimes if both the laptop and the amp are plugged in you can get a buzzing sound, this is caused by creating a loop but can be eliminated using a "ground loop isolator" such as
This one
You might not get a buzzing sound, I know I do, I run my laptop through a minidisc walkman before it gets to the amp to get rid of the interference. But my laptop was stupidly quiet, so I was using the walkman to boost the sound, it was only later i found it buzzed without it.. so check first, else you'll look a tit if it buzzes on stage and theres nothing you can do about it (the sound goes away if you unplug the laptop)