Can't Install Packages ubuntu 12.04

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http://askubuntu.com – I'm trying to install software by using apt-get and ubuntu software center UI, when I try the ubuntu software center it gets stuck saying installing and installs nothing and when I try the apt-get it gets stuck at the point "0% [Waiting for headers]" I tried modifying sources.list file. I installed ubuntu this morning. Do you have any suggestions for me ? Thanks in advance for your time. edit : After a while, I get the error 502 bad gateway (HowTos)