Get it here.
There’s plenty of documentation and examples on the website plus a fairly busy forum you can search or join. If you’re just dipping your photographic toe into the image-editing waters, you just cannot beat free.
Mind you, RawTherapee is not the same thing as a very full-featured product such as Adobe Photoshop, and doesn’t claim to be. However, used in tandem with open-source GIMP (which is a Photoshop mimic), you can create some very respectable works without emptying your wallet/purse.
Additionally, you can also give Irfanview a go.
It’s a great, tiny-footprint program, which has been around for over a decade. Also free, it offers many high-end effects and can even work with many of the free filter packs online which are available for Photoshop. Be sure to download the installer for your operating system, plus the “plug-ins” pack which corresponds to it.