1) I would suggest you to get a Linux distribution like RedHat or SuSE. If you don't want to spend money on them you can even download them for free. Have a look at
www.redhat.com or
www.suse.com. However if you decide to buy one, they both contain some good manuals which will help you with the installation and the basics.
Furthermore they both come with loads of software, basically everything you'll need for a start.
2) Perl, PHP mostly for webstuff.
3) As I said before, you'll find all the tools you need to get started on the CD's of the distribution.
4) You'll find loads of basic information on
www.linuxdoc.com.
Then there is of course google.com which will find the answer to many questions you mayhave. Also there is a lot of information about Perl, PHP etc. available on the web.