Whenever I read a script by a new writer, I can always tell if that person has read enough professional screenplays. I'm still learning, but when I go back to my scripts from college, I realize I hadn't read enough yet and I think I've gotten a bit better. Besides committing obvious format errors, new writers often misstep with their action and description.
First off, don't make it too long. we should never see giant blocks of text. Even if you do have a lot to describe, try to break it up. Use spaces to differentiate shots, places, people, etc. I usually break up anything that's more than 4 or 5 lines - and I think you'll see that's usually the norm.
Secondly, you want your action and description to flow well and not be clunky. I think it takes time to really master this. Newbies will often be too novelistic with their prose, telling us things the characters are thinking or feeling, or giving us backstory and exposition. Screenwriting books and professors often try to nip this in the bud by demanding that you never write anything that can't be shot - but then students get scared and start writing really wooden, lifeless description like, "Sarah enters. She sits down on the chair. Paul exits." I would read the House pilot to see something that veers on the edge of too novelistic, and comedy pilots like 30 Rock for what takes a more minimalistic approach. Focus on specificity and juicy verbs. Vary your sentence structure - and know that fragments are okay, even recommended.
Too much: "She stares longingly at the couple that has been together for 5 years, thinking that they're so lucky to be so much in love. She wishes she could be in a relationship like that.
Not enough: "She gazes at them."
Better: "She watches them, envious. So this is what love looks like."
The only way to get a sense of this stuff is to read, read, read. You can check out real scripts at Pilot School and Simply Scripts (linked at right), though don't waste your time with any transcripts at Simply Scripts - you want to read what the screenwriters originally wrote, not what a viewer has transcribed.
Reading also helps with scene pacing, dialogue, structure, etc. You should probably be reading at least a script a week, if not more.