It may help to compose an outline before you begin writing. Here is a sample outline:
The Electric Lamp vs. The Gas Lamp
1PP: Introduction and thesis statement
thesis: "The Electric lamp differs from the gas lamp in its history of development and its physical characteristics, and it is more efficient in its operation."
2PP: history of development
3PP: physical characteristics
4PP: operation (how it works)
5PP: summation and concluding thoughts