Download what is termed a "Live CD/DVD" most of the distributions have one, for example, Ubuntu Desktop. It's probably the easiest (Unless you know what you are doing?) to burn the iso on to a DVD. Put the DVD into your ROM drive and reboot the PC. Hopefully, your machine will have your ROM drive selected as the first boot device? Either way, you need to boot off the DVD, not your hard drive (Windows). Basically, it will run in your device's memory (You might need to select "Try Ubuntu") certainly do not install [yet].

Have a play, you can't do any damage unless you access your hard drive through Nautilus which is Linux's version of file explorer. You can read and write to your C: partition if you like so be a bit careful there until you know what you are doing or you could inadvertently damage windows.

If you like what you see but are not ready to fully take the plunge you can do what's known as a dual boot scenario if your hard drive is big enough. This, once installed, will give you a choice at boot time to run either Windows or Linux.

HTH