Thanks, I'll try to learn to use it!

First, thanks for all of the information. I really appreciate your help. As suggested, I'm going to post an additional photo and try to do it correctly.
I intentionally used a shallow depth of field, wanting to have a few in focus and the rest blurred . . . but I'm not sure if it looks as sharp as it should.

(Download)

Boy, so much information! Thanks for taking time to provide such a thorough response. I will try to use a lower ISO and increasing the aperture. The problem is with the horses, there's not much time to get the shot and I have to make sure that the shutter speed is fast enough but I'll give it a try.
I've made some notes on your comments and will try to put them to use. Thanks again for your time.

I am new to the forum and a new hobby photographer. My current passion is the wild horses in Arizona and our beautiful spring Mexican poppies. I'd really appreciate it if you'd take a look and then give me your feedback.
I recently switched from a Sigma 80-300 to a Canon 80-300 lens. I think it's helped with the clarity of my pictures. And, thanks to the info I've gained from following this forum, I've switched from multi-focus points to a single point. I've been using JPEG but started experimenting with RAW today. I'm currently using the software than came with my Canon Rebel for editing . . . I may need to invest in some different software.
Thanks for your time!

Canon EOS, Aperture Priority, 1/1600, ISO 800, EF 70-300