Finding Logitech Keyboard Drivers
The first thing that you'll need to know is the type of keyboard that you have. You should be able to find a name somewhere on the keyboard. It may be hidden away on the back near the model number, but it should be on there somewhere. If you can't find it, then don't worry too much. Things will just be slightly harder.
Go to the Logitech Support Page and click on the appropriate sub-section for your keyboard. There are three relevant ones: Gaming, Keyboards, and Keyboards and Mouse packs. Each section has a long list of product names. Logitech also provides images of each keyboard, so you shouldn't be in too much trouble if you can't find the name.
Pick your model, and then look at the top menu row. There are tabs for both downloads and troubleshooting. The download one should have a few different official fixes and a pack of software for the keyboard, at the very least. Choose your operating system, and then pick your download. Hit the button to download the software that you need.
If Logitech does not have the drivers that you need, for whatever reason, you can also check the Softpedia directory for Logitech keyboards. Type in your product's name, and you should be able to see it.