When you know how to ask. That's the key. Knowing how to ask, and knowing what to ask for. You can ask for anything you want.
But asking for a new car when you really want a Porsche, doesn't mean you'll be satisfied with just "a new car". And just saying "I want a Porsche" and then going on about your life as usual probably won't get it for you.
You have to focus your thoughts on getting that new BMW, mentally picture yourself in it, holding the steering wheel and driving around. You have to truly believe you deserve it and allowing it to come to you is the way to get it.
By articulating what you want activates the first level of the Law of Attraction into awareness and action. Feeling and acting as if you've received it is the second, knowing you deserve it and allowing it to come to you is the third.
Having doubts about your worthiness produces negative energy. Negative energy produces negative results.
You have to believe that the Law of Attraction works and you have to believe that you deserve what you desire to have the wonderful things manifest into your life.
Positive energy is the only way to get positive results. And positive energy is only produced by positive thoughts.