The best way to install is to use a LIVE edition. This is useful beacuse you have a nice installer intergrated and you can try it before you have to install the OS on the computer.
From there, if you come from Windows, I would raccomend KDE, as it is stable and customizable. Search “KDE screenshot” to see what it looks like, and if you like it.
Debian should also be lite enough for older machines, and it is the most stable distro I’ve tried. With this OS, there are already web browser, media player, office suite,… but you can also download Steam, emulators and lots of software