One: Rely on openSUSE packages, do not install rpms from outside the openSUSE infrastructure. Exception: Google Chrome.
So, if you are looking for software, first search Yast’s Software Manager. If the package isn’t available in there, go http://software.opensuse.org and search there. Mind, openSUSE defines all non-official repos as unstable. You will see, that Filezilla is in the network: repo. A one-click-install button shows in the search results.
Two: flashplayer is a different case. A repo is offered throug Yast - Software repositories - Add - Community repos, but personally I’d rather go with the version in the Packman repo. That one is in the same list as Adobe’s repo.
Three: I don’t know what Atom is.
Atom is some type of text editor but it does not look like anyone has built it for openSUSE So you would need to obtain the source from github and build it.
You probably will want to know all about setting up multimedia to watch videos, play music, and so forth on your system. This also helps take care of the Flash setup.