MP3 files are the most common music file used, but AAC files are growing more popular. It is now easy to convert those MP3s into AACs with a free program called Free MP3 WMA Converter.
Download and install Free MP3 WMA Converter (see Resources below).
Open the program. Go to "File" and select "Add files."
Find the MP3s you want to convert, click on them, and click "Open." This will send the files to the Files to Convert list. Continue doing this until all the files you want converted are in the list.
Change the output format to AAC. Do this in the Output Configuration section, below the Files to Convert list,
Click the "Convert!" button in the lower right-hand corner once the list is complete and the settings are correct.