Difficult to know how to help if you don't say what you've tried.
If you plug your USB cable between the phone and the PC, select "Disk drive" when the phone asks you to choose a connection type", and open a file browser (Windows Explorer if you're running Windows), you can then drag and drop MP3 files from the PC to your phone. I would suggest making a separate folder for the MP3s, and that folder can be named anything you like and the Music app will find them.