Hello,
Download the driver here:
http://support.logitech.com/en_us/product/hd-webcam-c615#download
From there, you should just launch the program you downloaded and it will guide you through the installation.
In the future, you may find it faster and easier to google the product name you want to find a driver for (IE, Logitech C615).
Find a link that takes you to Logitech's website. The one I used was titled "Logitech C615 - Logitech Support".
There you will wanna try to find something on the device's support page that says "Downloads" or "Drivers" or something along those lines. Don't be afraid to click around.