LOL macs are good computers so i'm not gonna have a go at you for that, though in this instance the questions you've asked it's irrelevant that you have a mac
OK first off you seem to be a little confused about memory and hard drive space. see link below for an explanation
http://www.theanswerbank.co.uk/Technology/Comp uters/Question579000.html
OK so yes if you have a lot of music it probably is all your Itunes music that has filled up your hard drive, an external hard drive is just as it sounds, a hard drive that is in separate box that you connect to your computer via USB which would let you move some of the information on your internal hard drive over onto it and therefore freeing up space on the internal one
The popup you got was probably advising you you where getting low on space on your system (internal) hard drive and asking if you wanted to clean the drive up, this should remove any unused stuff and temporary files to free some space on the internal drive, it will not create more space and keep everything as thats not possible, the drive is a fixed size and can't get bigger.
When you get your external hard drive I would go through the system and move anything that you don't use on a regular basis onto the external drive (like I bet you don't use all you music on itunes all the time!) this should free up some space on the system drive and get the computer running smooth again.