Platforms (or starting a scene)
Platforms are the start of the scene, and are where you establish what your scene is about. The important thing to remember is that you don't have to be interesting or funny as soon as you hit the stage. What you should be focusing on is building the world in which your scene is going to take place. The richer the world you build at the start of the scene, the easier you will find the rest of the scene which follows.
Below are some suggested ways you can start a scene. None are necessarily better than the others - it is important to find an approach which works for you. What they do have in common is that they are about creating a world from which the rest of the scene can then emerge.
Establish the who, what, where (who your scene is about, where it is occurring, and what is happening in it).
Bring one strong thing to stage (a voice, an emotion, an attitude, a physical trait).
Establish the main relationship in the scene (how do you relate to the other characters).
Hero and quest (who is the main character of the scene and what is their quest)
Explore your environment (flesh out the physical world of your character).
One final suggestion is to start your scenes with positivity. Performers have a habit of immediately picking fights in a scene, in the belief that conflict is entertaining. What is often more effective is to start the scene without conflict, so we can get to know the characters and, more importantly, begin to care about them. It also makes any bad things which eventually do happen to them far more effective.
Be changed
To tell an interesting story, or to create an interesting scene, it is important that you allow yourself to be changed. This is because stories are about characters and relationships being changed by what happens to them. Cinderella changes from servant to princess, Hamlet is brought low by his desire for revenge.
These stories would not be as interesting if the characters remained static and unaffected - if Cinderella decided it was too difficult to go to the ball or Hamlet decided that he could cohabit with his Uncle. As an improviser, if you want to tell an interesting story you need to be affected by what happens on stage. If someone dies, this should fill you with grief, or anger, or joy. If someone offers you a cream bun, this should shock you, bother you, or delight you.
So when you are in a scene, focus on your fellow performers and let their actions affect your character - physically, emotionally, and intellectually.
Be bold
Many of the principles of impro so far described have been responsive: as a performer you need to be aware of other performers, accept their offers, be changed, and look for the patterns in what is already on stage.
But this is only part of the story.
Often the best thing that you can do to support your scene partner is to bring a strong and clear offer to the stage. This doesn't mean taking control of the scene, or killing off other people's ideas. What it does mean is that to be a skilled improviser you need to learn how to inspire your team mates.