Can I use a song title in my story?
Yes. It is perfectly acceptable to use the title of a song if you want to say what your character is listening to or singing, but you cannot write the lyrics. The title is not copyrighted, but the lyrics are.
Can I use a real brand name/shop name in my story?
Yes, it is fine to use a brand name such as "iPod" or "Coca Cola", or a shop name such as "Sainsbury's", but what you cannot do is misrepresent the company/brand or make out that you've created it yourself. Mention it, but don't abuse it.
What is a Mary-Sue?
A Mary-Sue is basically a 2D character. They are shallow, have little personality and everything seems to go right for them. Usually they are also perfect in appearance, or a carbon-copy of the writer.