I used to think that in order to write a scene, a chapter, or even an entire story, an author had to write it from beginning to end, but after trying this several times, the frustration became unbearable. When it comes to writing, begin with the element that comes most natural to you.
Dialogue is usually the element that comes the easiest for me. I'm not completely sure why, but when writer's block rears its ugly head, I simply begin to type the conversation that I want to happen between my characters. I am always amazed what can happen after I fill a few pages of dialogue. All of a sudden, the plot becomes clearer, the setting finds it's way into the scene and conflict reveals itself as my characters speak to one another.
Most of my ideas for dialogue are from actual discussions that I've had with someone or (one of my favorite things to do), a result from eavesdropping on the conversations going on around me.
Here's a little writing exercise: Go to a public place, whether it is a coffeehouse, grocery store, or even a local sporting event. Bring a pen and paper and write down snippets of what people are saying around you. Then, take a look at what you've heard and put the dialogue into a scene. It could turn into something really fantastic.