MP3 music files won't always work on some older CD players. You'll have to "burn," or transfer, your songs specifically in an audio CD format in order for them to work in older CD players. To convert MP3 files onto an audio CD, you'll need CD burning software, available to download for free online.
Download a free CD-burning program from the Internet. "Express Burner" is an example of one that will work in Windows (see Resources). Click on the big "Download Now" button on their website, then hit "Save As" when the download window appears. This will download the installation file to your computer.
Open the installation file after it's all finished downloading. Pick a directory for the program and install the program on your computer. Open the program by double-clicking on the shortcut icon on your desktop, or by clicking on the "Start" menu and finding your program under "All programs."
Click on the "Audio CD" button in your program. Press the little button in front of the CD tray to open it, and insert a blank disc into your computer's CD drive.
Click on "Add songs" to add the MP3 songs you want one at a time. Keep track of how many megabytes you've added to make sure that it's under the amount of space you have on your blank CD.
Leave at least a few megabytes of space, as the software program will often need this to complete the burn. Click the "Burn" button to create the audio CD. After it finishes, you can put your new CD right into your CD player and use it.
It's often cheaper to buy blank CDs in larger numbers from the computer store.
DRM stands for "Digital Rights Management," and if the MP3 files you have came from a website that puts DRM on their songs, then you may not be able to burn your MP3 to an audio CD at all. Unfortunately, there is no legal way around this.