How do I start an e-commerce business plan?

Seven parts of a plan are as follows.

Executive summary - which is a summary of what exactly you intend to do.

Company overview which is an overview of company operations.

Market analysis which is an overview of the market analytics you want.

Products and services which is an overview of what services and products you intend to supply.

Marketing plan which is an overall view of what you need in terms of advertising, reach and conversions.

Logistics and operations plan which is a full plan on what your shipping and transaction network will be.

Financial plan for your taxation, services and operating costs.

Here's a nice site with a breakdown of the types of business models you can look at