You need an MP3 shield, one that allows you to enter the track number and play it. You can do a search for Arduino MP3, and your mostly likely going to either get a shield or a small chip like board. If the vender is nice, they will have a sample code you can look at before you buy the part. Shields tend to be the easiest to program, but if you already have multiple things running off the Arduino, your more likely to have complications with pin usage. So check the schematics and data sheets on what pins the shield takes up before hand.