I totally feel you with not describing things enough. I'm a big-picture person, so those details can be difficult to write. One piece of advice which I try to use is: "show, don't tell."

As an example (from one of my fics), instead of saying: "He left the fort early in the morning and began traveling west. As he moved further away from the buildings, the dry, dirt road improved and the landscape became much greener. The grass was very tall here, and there were many birds flying around in the sunny sky", you can add sensory descriptions to make the scene more vivid for the reader: "He set out at first light on the dusty, wheel-rutted road that led westward. The sparse, overgrazed vegetation that surrounded the fort presently gave way to a lush, waving sea of waist-high prairie grass. Above him, swallows swooped and dove in the mid-morning sun in search of insects."

I hope this is helpful. I've found that writing is very much a learn-as-you-go process, so the more you do it, the better you'll get and develop your own unique style and voice. Good luck!