If it has USB, buy an external USB soundcard with a line input. If the laptop runs Windows there is plenty of choice on ebay, for example those branded 'Creative Soundblaster'. Make sure it has drivers for your version of Windows.

I used a 'Terratec Aureon 5.1 USB soundcard' (which is one a the very few which runs under Linux) when I had to record on a laptop which didn't have a line-in socket. It had several inputs and its own headphone socket and volume control.