Unlike novels, there shouldn't be any mention of what the character is thinking in a screenplay. Everything must be conveyed through either the character's actions or in their dialogue. Otherwise, leave it out. The camera can't see what's going on in their mind, so the best thing to do is find a way to show it. For example, if the character comes home from a dreadful day at the office, have them slam a door, throw their work bag across the floor, take an aspirin, etc. Talking about things in dialogue is okay too, but the old 'show, don't tell' motto applies in screenwriting. If something can be shown through some sort of action, it's better than saying it. On format, everything should be in a regular font. Italics are rarely used in scripts. If emphasis on a particular word is very important, it can be underlined, but don't over do it. Hope this helps.
P.S. There are sites like DailyScript.com and others that let you download free scripts to actual movies. Some posted on there are shooting scripts which are a little different than spec scripts, but they might help give you an idea of what to include or leave out in terms of character and description. Also, the book "The Screenwriter's Bible" by David Trottier is really great for learning the proper screenplay format.