What's the best way to learn how to properly adjust photos in Lightroom?

Polarr has an interactive photo editing guide here https://www.polarr.co/guide to help you understand how to adjust photos, while you can also use  their photo editing app for free https://v2.polarr.co/

Scott Kelby's book is great, and Julianne Kost's videos have helped me. I also like Martin Evening's The Adobe Lightroom 5 Book - the Complete Guide for Photographers. Another good resource is Adobe Lightroom 5 the Missing FAQ by Victoria Brampton. It has a quick start section to get new users going and throughout the book there are in-depth details on how to do things.

Once you know the basics via something like Scott Kelby's book, I found the videos by Serge Ramelli really helpful. His videos are easy to follow along with but produce very dramatic results. He posts a series of free videos on his youtube channel, and if you want a more comprehensive training it's well worth the money to buy his complete Lightroom series on his site http://photoserge.com/. He seems to regularly run 40% off specials, so wait for these.

In order to adjust exposure, there are several techniques that can be used in the Develop section of Lightroom. You can adjust exposure as a whole in the right hand column, but also selectively do so by using graduated filters, found in the upper right side. These graduated filters allow you to also adjust hue, color temperature, as well as most other settings in a concentrated area. I personally like to use graduated filters to expose parts of my photos that are underexposed. For example, if my subject has some backlighting from the sun in the sky, they are often underexposed, and I proceed to fix this by using a graduated filter. I can also make other small adjustments, including clarity, color temperature, saturation, etc. I try to adjust exposures in the photo so it all looks balanced and not uneven, as I like to maintain a natural look in them. Saturation and vibrance allows your images to look more vivid with color, as you can see when you adjust the bars. I also like to use the clarity option, which intensifies areas of your photo, and makes it sharper. There is also an option of lens correction, which can correct aberrations and vignetting from your lens. The last tool I often use is the cropping tool, which allows me to straighten horizons and crop out parts of an image I don't particularly like.