Instructions
Using iTunes
Download and install the most recent version of iTunes.
Open iTunes.
Click the "File" tab and select "Add File to Library" or the "Add Folder to Library" option. The "Add to Library" window will open. Navigate through your folders and select the albums or songs you want to import to iTunes. Each track will be imported to your iTunes music library automatically.
Select each MP3 file you imported to iTunes.
Right-click the highlighted files and select "Create AAC Version." ITunes will convert the MP3 files to AAC files and save them to the iTunes folder on your PC.
Using MP3 to AAC Converter
Download and install MP3 to AAC Converter.
Open MP3 to AAC Converter.
Click the "Add" button and import the MP3 files you want to convert to AAC format.
Click the "Output Directory" button and choose a destination folder for your converted music files.
Click the "Convert" button.
Using DVDVideoSoft Free Audio Converter
Download and install DVDVideoSoft's Free Audio Converter.
Open Free Audio Converter.
Click the "Add Files" button and select each MP3 file you want to convert.
Select "AAC" from the "Formats" drop-down menu.
Click the "..." button to the right of the "Save to" field and select a destination folder for the converted music files.
Click the "Convert" button.