The beginning in the screenplay is designed to accomplish a certain set of goals, so when you are considering how to start a good screenplay you really have to keep these in mind otherwise you are going to negate the purpose of this beginning.
First, it is supposed to make the audience identify the main character, or characters, so that they will be able to follow them through the rest of the screenplay.
Second, it is supposed to identify their main character needs and traits so that the audience has a relative idea of who they are and, in a sense, why they are going to be doing what they are doing. This does not mean that you have to leave everything open and spelled out, but they have to have a context so everything does not just seem out of place.
The beginning is also where you are going to identify most of the main supporting characters that are going to work through the rest of the screenplay. Some characters show up in the second or third acts, but the main supporting characters should all be identified in the screenplay beginning so that we know how they are going to relate to the main character. We should essentially know this character, though they can still surprise us. We should have a basic idea about their character, their history, and their drive so that when they encounter the problem at the end of your screenplay beginning you will have an understanding of what they do. You may want to also write character biography when you are trying to figure how to start a screenplay. This is going to help you to create a clear idea of your main character, or characters, and will allow the beginning to move into the second act.