Usually used in screenwriting [http://screenwriting.tumblr.com/], the 3-act structure can also be used in creative writing - novels and short stories. It can give your writing good dramatic structure while still giving you the freedom to create unique worlds, plot and characters. The 3-acts of your plot are the beginning, middle and end.

ACT 1

(protagonist) - introduced and personality shown.
(key line) - Spoken by a character which will give the audience a clue as to what themes and ideas will be explored in your story.
(hook) - Grabs the reader's attention and draws them into the story.
(o) - Usually focuses exclusively on the protagonist, deepens their characterisation by showing their personality. The interaction with the situation you have designed for them will form the character problem which they must overcome by the end. Acts as a benchmark by which the audience can measure character growth. Major subplots are also established.
(turning point 1 / climax) - Act wound up by introducing an element of risk. Incident sends story in new and dangerous direction. Alters protagonist's motivation / goal. Deadlines if needed usually established here.

ACT 2

(a) (o) - Audience chance to relax after climax. Protagonist reflects on dilemma. Has now formed a goal, but it will prove a false one. There will be a setback, but minor in grand scheme. Problems that occur later are set up here. Section ends with:
(focus point 1) - Refocuses action begun at TP1. Also sign of growth in protagonist.

(b) (o) - Protagonist takes first decisive action towards attaining ultimate goal of story. Similarly, any subplots will show the first signs of change in the protagonist's character. Obstacles get tougher and protagonist gets stronger.
(point of no return) - Protagonist was floundering as the victim, uncommitted and not in control. After PONR, the protagonist begins to take charge of their own destiny. PONR is moment of realisation for protagonist, which forces them to reassess their quest and consider giving up, but ultimately continue, with no going back to their former ways and notions.

(c) (o) - Subplot sometimes explored here or location changed. If so, this section may include a self-enclosed sequence, something that could be a short film. Also shows protagonist holding to commitment made at PONR. End's with:
(focus point 2) - Refocuses story by restating problem at hand. Good focus points should be linked, with the first setting up the second. They also emphasise protagonist's character growth, first FP hinting at it, the second revealing it in full.

(d) (o) - Sometimes be a long, expositionary speech explaining the inner motivation of the protagonist. Section mainly sets up action that will lead to:
(turning point 2 / climax) - Refocuses story. Thrusts into next act. Well integrated into the story with obvious set-up, will lead to a sense of failure / despair.

Fast summary of Act 2:
(o) - (focus point 1)
(o) - (point of no return)
(o) - (focust point 2)
(o) - (turning point 2 / climax)

ACT 3

(o) - Brief pause to relax. Focused entirely on accelerating action toward final climax. Often has chase or pursuit. Act 3 is often a single major sequence.
(climax) - Protagonist faces final confrontation. Peak emotional moment for protagonist. Provides satisfying ending.
(epilogue) - Ties up all loose ends. Key line or image hooks set up at beginning will be resonated, but with their meaning changed.

THE END

Using this as a guide, your creative writing should seem far more professional.