how can I to un-install a program?

Most programs (Applications) can be dragged from your Applications folder to the trash, and that will effectively delete them, with the exception of some remnants that no longer have a part to play.

These applications are usually installed simply by dragging them from a disk image into the Applications folder.

The other kind of installation is where applications require some ancillary files to be written to deeper parts of the operating system, and these will usually come with an installer. Such an application that is well written will also come with an uninstaller, to remove all of the bits and pieces that were initially placed. For such applications you would use its uninstaller.

In summary, how you remove an application will mirror how you installed it to begin with.