How to change the post form from plugin?

If you want to change/add something to post form, you have to deal with meta boxes. I would recommend you to read through these articles:

• Reusable Custom Meta Boxes Part 1: Intro and Basic Fields
• Reusable Custom Meta Boxes Part 2: Advanced Fields
• Reusable Custom Meta Boxes Part 3: Extra Fields
• Reusable Custom Meta Boxes Part 4: Using the Data