Ubuntu Software Center is a GUI made to help users install and uninstall stuff on their Ubuntus, but is very heavy and cumbersome.
The Synaptic Package Manager is better and less intuitive. No alternative to that. The lightest path to install/uninstall is the Terminal, but it is geekier.
When you are lucky enough to have a program package in the .deb format it will usually open in Ubuntu Software Center. Too heavy for Lubuntu, which still uses the very light Gdebi Package Manager, a small utility to install deb files. When I installed LXDE desktop environment to check it out (see my page "What is Ubuntu / Kubuntu etc") I discovered Gdebi as the main installation GUI. To get it open Synaptic Package Manager, search "gdebi", check it right-click for installation and apply.
And after that it would be a good idea to associate deb files with Gdebi.