If you want to be a screenwriter then the key is very much practice and one of the core pieces that you can be practicing is a scene. A movie of course is made up of a collection of scenes and that is why focusing on how best to put one together is going to be essential for you to improve and reach a point where you will be able to create a larger project. Gia Walsh Bronxville writer and a highly respected member of the film community is someone who started out creating scenes in her bedroom before catapulting herself to stardom and these are some of the activities which she used to do in order to create a great scene.
Many people find dialogue to be difficult because it is quite odd to actually write speech. Naturally most scenes are going to heavily depend on written speech in order to bring home the emotions and the mood of the scene, so this is a great place to start practicing. Instead of just sitting and staring at an empty page trying to think of how people speak, you can instead look at actual speech and rewrite it. For example record a conversation between you and a friend and then when you get home play the conversation and write out the dialogue. This is going to be great for your dialogue writing abilities.
Another great activity which you can do that is going to significantly help you with your ability to write a great scene is to rewrite scenes from TV shows or movies. Alternatively you can look at writing your very own scene, but using the characters from your favorite TV show. The reason why this will help is because a great amount of the work has already been done for you in terms of the creation and the development of the characters. This will help you to focus on how the scene plays out, what the mood is and how to get from the start to the end and achieve what it is that you want to achieve for the storyline.
Many scriptwriters will say that every scene has to have a certain structure and they will map out how the scene is going to go. First of all think about what the scene is going to achieve, how is the scene going to help the storyline. Next up think about where the scene is starting and what should be happening by the end, this gives you the parameters within which to build the scene. Then think about how the scene looks, where is it set, what props are there, how many people are in it and whether or not that reflects the mood and the emotions of the scene. Finally write out the dialogue and consider how it will all be delivered.
The key to creating any large story or movie is to focus on an individual scene, from there everything will grow.