The answer to this question depends on what kind of business you wish to start. There are many paths to starting a small business. However, I will give you some general information.
You can start by looking up the laws in the location where you live. Find out what is required to start a business in your area. If you live in the United States, you might have to check the laws of both your state, and your city or town.
Once you are aware of the regulations and registration requirements, you should write a business plan. There are many books and free articles online to help you write your plan. Mainly, you want to highlight the purpose of your business, your target customers, how you will reach your customers, and estimate how much you can make. If you plan on getting a loan or having investors then you will need a more formal business plan. If you do not need financing, then you just need a plan for your own reference.
Seek out organizations that can help you with your business goals. For example, in the United States, there is SCORE which is the Service Corps of Retired Executives. This organization offers free advice to people wanting to start small businesses. They may also have funding available if you qualify.
If you know someone who has started a small business, you might ask them for advice as well.
So in short:
1. Find out the legal requirements
2. Ask for advice
3. Write a plan
4. Act on the plan (register the business, obtain the equipment you need, begin marketing, etc.)