Learn the basics of your form. It is impossible in one article to really lay out all the basics of screenwriting, playwrighting, and television writing. Suffice it to say, each form takes a little getting used to and each form has it's own "essentials." I would suggest anyone who wants to begin any type of script writing consult an expert in the field--whether through a how-to book, a workshop or a class (you need not get a degree in scriptwriting; one class should give you a good look).
Of course, anytime you purchase a book or enroll in a class or workshop, employ the "buyer beware" mindset. A lot of people out there know how much other people want to break into scriptwriting and take full advantage trying to sell you "secrets of the trade" sure to "get you in the door." In my opinion, there are no secrets that are going to get any writer in any door. People break in for all kinds of crazy and random reasons. The one thing you want to worry about at the start is writing the best possible script you can. So, if the book or class looks to be selling any golden tickets, save your $14.95.
On the other hand, don't dismiss all you can learn from a good expert, teacher, or mentor. There are definitely people out there--some of them unproduced, some of them quite accomplished--who have a lot to offer. I have teachers whose words of wisdom I still reflect on every time I start on (or get stuck in) a new project. Choose your teacher wisely. Look for people who will read your work and give you personal feedback. Look for people who have some kind of scriptwriting background, be it professional or academic. Look for classes and books that emphasize the basics--finding ideas, developing story and conflict, and developing character. Finding the right screenwriting resources can be a tough task, like finding a teacher or "how to" book in any subject. But when you find the right one, you've hit the jackpot.
As for whether some "rules" taught by teachers or books are too constricting, I'm a big believer in learning the rules before you try to break them, walking before you try to breakdance. And often, the "rules" are more like laws of nature--things that you really just need to have to make a script work.
Also, read many samples of the form you wish to write in. In other words, read scripts like the one you wish to write or by writers you admire. This is often the best way to figure out how scripts are put together, maybe noticing basic things you might not have even considered, like how long a script should even be. Your average 120-minute screenplay, for example, will be 120 pages long. Your average TV drama teleplay may be 44-50 pages long, not 60. Stageplays can range from the very short (ten pages) to 120 pages and several acts. Just be careful in the case of reading produced works as the script you read may be a "shooting" copy and not a draft straight from the mind of the writer(s).