3.
Ad 1
In my opinion the easiest way to learn English is to surround ourselves with the language including: listening to English radios, watching series and movies in English, reading books in English and joining English forums about topic we are interested in.
Ad 2
I think that learning vocabulary from songs may be difficult because many artists use slang and not always stick to general grammar rules in their song. Therefore, it is easy to learn something that is not used in official language accidentally.
4.