You also have to run apt-get update after changing the sources.list file. Then you can get the applications via apt-get install "package" you can also remove them. with dpkg -s "package" you can see if something is installed,... try for example "dpkg -s apache"
But I strongly reccomand you to read the faq, it's well explained on the debian page. And it knows muche more about the errors and how things work than I do. Also check the man of dpkg and apt-get, it's very usefull.
really, you'll learn a lot about linux and you'll even understand what ye're doing. At first, it'll take some time but once you know, you won't forget.