The first step to troubleshooting this issue (and many others) is to be sure you have the latest version of iTunes installed. From the iTunes menu bar, select iTunes > Check for Updates.
Next, use Font Book to determine any corrupt fonts, or to disable any third party fonts (any font not installed with your OS). To do this:
- Open Font Book.
- Select any fonts with warnings, then from the Font Book menu bar, select File > Validate Font.
- Once you have validated your fonts, you may want to turn off some fonts or font families. To do this, select the font or font family from the Font list (or an entire collection from the Collection list) and from the Font Book menu bar, select Edit > Disable (Note: Select the proper disable command).
- Once you have finished, restart your Mac. The iTunes Store should be readable again. You can enable fonts by opening Font Book, highlighting the font you would like to enable, and selecting Edit > Enable.