You questions seems to be a bit too broad,
I will limit myself to answer the question in your title,
What to do after installing ubuntu.
There is a great website that published this kind of posts everytime a new version is released,
And this is what I have actually used mysel, back then...
It answers your question about Codecs:
13: Install Restricted Extras:
There are some Restricted extras available in Ubuntu repositories
which can't be install while installation but you can install these
Restricted extras by yourself. Restricted extras contains: mp3
playback and decoding, support for various formats, fonts, java, flash
plugin, lame, dvd playback. Terminal Command:
sudo apt-get install ubuntu-restricted-extras
14: Codecs and Enable DVD Playback:
If you are multimedia user and use Ubuntu for multimedia then these
codecs are very useful for you. You can install them by single
command. Terminal Command:
sudo apt-get install gstreamer0.10-plugins-ugly gxine libdvdread4 totem-mozilla icedax tagtool easytag id3tool lame nautilus-script-audio-convert libmad0 mpg321 libavcodec-extra
Following command for Enable DVD Playback: Terminal Command:
For Ubuntu 14.04 Trusty install more codecs: Terminal Command:
sudo apt-get install libxine1-ffmpeg mencoder
15: Enable H.264 support for Firefox (Only for 14.04 Trusty):
FFmpeg plugin for GSreamer is dropped from official Ubuntu 14.04
official repositories, Firefox doesn't support the H.264 codec. So you
can install it from third-party repository.
Enter following commands to install codec: Terminal Command:
sudo add-apt-repository ppa:mc3man/trusty-media sudo apt-get update
sudo apt-get install gstreamer0.10-ffmpeg
It answers your question about Compression Tools:
16: Compression/Decompression tools:
You can compress and decompress 7z, zip, gzip, rar, xz, tar, bz2, xar,
tar.gz, tar.bz, and many others.
Enter following command in terminal to install in your Terminal Command:
sudo apt-get install p7zip-rar p7zip-full unace unrar zip unzip sharutils rar uudeview mpack arj cabextract file-roller
Regarding Bittorrent, I recommend you to install qBittorrent, you won't need anything else.
sudo apt-get install qbittorrent
And regarding your last question, about music tools
You should look into Ubuntu Studio,
Even though it is a distribution in itself, I think you can just install all those packages, this tutorial may help you
well, after all I did answer all your questions