Python is the most minimalist without being cluttered by unnecessary features (Ruby) or too many functions in the default namespace (PHP). The syntax is very elegant and easy to read (unlike Perl).
I recommend that you read Learn Python the Hard Way(http://learnpythonthehardway.org/index).
1. if you are making web apps then Google App Engine / Python is a good choice
2. If you are developing Ipod/ipad/iphone touch apps then you should learn obj C
3. If you are more scientifically inclined and working in bio / Chem field then again I would recommend python.
4. If you are you young and have ample spare time at your hand then learn C.
5. if you are just plain beginner with no clue then I would suggest Ptyhon again.
http://diveintopython.org/toc/index.html or http://www.ruby-lang.org/en/documentation/quickstart/
"what each language is specifically used for when developing"
They're used for all kinds of things. Not worth worrying about IMO.
Python is structured, clean. Ruby is too freestyle for a beginner (imho), perl is a shipwreck.. and PHP is too limited in scope