If you actually need the raw audio data: http://stackoverflow.com/questions/938304/how-to-get-audio-data-from-a-mp3
Note that the byte buffer required will be significantly (2-3 times) larger than the mp3 file itself. To calculate size required, multiply samples in the file by bytes per sample:
long bytesReq = stream.getFrameLength() * format.getSampleSizeInBits() / 8;
Depending on the required functionality and how lazy you are, you could avoid large storage of data and stream it in as you need it, as the SO answer above returns an AudioStream.
Also, Danny's advice is good advice.