Your first mistake was spending money on a Macbook lol.
And to clarify your problem, your transferring music files, etc from your current PC to the Macbook via Flashdrive and having difficulty with space. Are you having difficulty on the Flash drive or just the macbook?
On your PC, open the properties menu of the flash drive. It will tell you how large the drive is and what space is left. Post your results.
Before you continue, I would recommend purchasing an external hard drive for music files etc. Download the music files to the ext HDD and you can plug and play between computers and not interfere with the space on the macbook. They are relatively inexpensive.