I’ve got my stereo headphone socket to my line in on a laptop
Are you sure? Most laptops only have mic-in and headphone out. The mic input will “work”, but it’s about 100 times too sensitive, it’s usually mono, and it’s often noisy.
A line-input works with line out (The RCA outputs on a tape deck or DVD player, etc.) and it also works with a headphone otput.
If you have line-in and the signal is still “too hot” reduce the recording level (the microphone icon in Audacity). 0dB (1.0 or 100%) is the “digital maximum”. Your analog-to-digital converter will [u]clip[/u] (distort) if you “try” to go over 0dB. If the recording is distorted, lowering the volume after recording won’t help.
It’s usually best to shoot for peaks between -3 and -6dB so you have headroom for any unexpected peaks. Nothing bad happens when you get close to 0dB, only if you try to go over. And, you can go quite a bit lower with no issues. Then, you can Amplify to bring-up the volume after recording.
You can use a desktop/tower computer with a regular soundcard (or soundchip) or you’ll need a USB audio interface with line-in. The Berhinger UCA202 is popular and inexpensive. (Don’t get a regular “USB soundcard” because they are like laptops with only mic-in and headphone-out.)