If you are a novice then it is unlikely that you've changed any of the default settings. Enable the menu bar in iTunes if hidden with Ctrl+B, then use Edit > Preferences > Advanced to look at some options that control what happens when media is added to your library. I would expect to see the media folder listed as C:\Users\<User>\Music\iTunes\iTunes Media and for the two options Keep... and Copy... to both be ticked. If so that is a good start as it means your library is in the standard layout which is portable.
With iTunes closed you can copy the entire iTunes folder from C:\Users\<User>\Music\ to the top level or root folder of an external drive to make a working copy of the library. You can access this new copy of the library by holding down shift (Window) or Option (Mac) as you launch iTunes. You keep hold the key until a prompt appears asking you to choose or create a library, the browse to an open the iTunes Library.itl file that is inside the iTunes folder.
A complication arises when switching from Windows to OS X in that most external drives for Windows come formatted as NTFS which OS X can read, but cannot write to without additional software. iTunes needs to be able to write to the drive in order to update the library. The options for getting around this obstacle are to download an NTFS write driver, or copy the iTunes folder to a second external drive formatted as HFS+ which is the default Mac filing system (and which Windows won't read or write). Once you've made the copy onto HFS+ and made sure that your library works properly there, and you've migrated any other data that you want to copy over from the Windows system, you could reformat the NTFS drive for use with the Mac and copy the data back to it so that you have a backup.
Another option is set up your external drive on the Mac to begin with, connect both computers to the same network, share the Music folder in Windows, and copy the iTunes folder from the Windows system over the network directly to the external connected to the Mac.
tt2