how do i record a lp or casset to my iMac

You will need appropriate conversion software. I used 'Spin Doctor' which was a part of a bundle of software that came with Roxio Toast. It was not the most stable so search for better alternatives.

You also will need to get a cable that will connect to the audio outputs of the record player or cassette deck, usually RCA connections. On the other end will require a plug that is compatible with the audio input port of your Mac. This will be a 'Y' connector.

Ciao.

If your iMac has a Line-Level-Input jack, then you could connect the line-level-output from your player to that input, and using an application such as Audacity to record and save the recording as an audio file.

http://audacity.sourceforge.net

Also there are companys that have players that you can connect via USB --

http://www.ionaudio.com/products/turntables

It looks like it will have to be either the dual microphone inputs or a USB connector on your iMac. I have a Macbook Pro with an audio input port which makes life a lot simpler. In this case I am not your best advisor, but clearly these are the two avenues available to you. I would speculate that the microphone inputs would be the simpler option.

Ciao.