It can be hard to know what topics you should write about. This can be true regardless of whether you want to start a completely new business or if you want to build a website based around your existing business. If you want to build a website around your existing business you need to research which topics that are likely to give you the highest return. 
It can be a good idea to write about all topics that can bring you visitors to your website but your priority should always be to add the articles that are likely to give you the highest ROI first. Once you have covered all high return topics you can start writing articles about topics that are likely to provide a lower ROI.
If you want to start a completely new business then you are free to build that website around any topic and you need to find a topic that can give you a high ROI on your investments.
The methods used to find high ROI topics are the same regardless of whether you start a new business or build a website around an existing business.
Popularity
The first thing you should consider is the popularity of different search terms. The more popular they are the more traffic they can bring. You should primarily target topics that can bring you a lot of traffic but you will also have to consider other factors. Not all traffic is profitable and it does not matter how much traffic you get if that traffic is worthless. If you are unable to make any sales from that traffic.
Competition
Some topics are very competitive because hundreds of other websites try to rank for them. Other topics are a lot less competitive. The number of websites writing about a topic says very little about how competitive a term is. You have to look at how well optimized the top ranking pages are and how many links they have pointing to them. Is the top ranking pages index pages or sub pages. It is generally easier to outrank sub-pages then index pages. There are plenty of guides online that can teach you more about how to evaluate how competitive a term is.
Binary options is an example of a very competitive term. If you want to reach the top 5 for this term you are going to need links from more than 1000 different websites. https://www.binaryoptions.net/ is currently on the bubble of reaching the top 5 (6th place). That website has links from more than 1200 different websites and are still not able to make it into the top 5.
Shaving kits is an example of a very uncompetitive term. If you want to reach the top 5 for this term u are going to need links from less than 10 different websites. https://www.groominglounge.com/shaving/shaving-kits that ranks third for this topic does not have any links at all pointing towards it.
Commercial intent
The term commercial intent refer to an intent to perform a commercial act based on the search. IE that someone is looking for the information they need to buy a goods or service.
Some terms are associated with commercial intent. Others are not. Terms that are associated with commercial intent are generally a lot more profitable then other terms.
You should primary target terms associated with commercial intent. The higher the commercial intent the better.
One visitor that is looking to buy something is worth 100 visitors that are not.
Motivation
It is also important to consider the motivation of the buyer. A buyer that is motivated to solve a real problem in their life is willing to spend more money and try more options than some one that is looking to buy a luxury product.
Good examples of this includes acne, hair loss and cancer treatments. The buyer is very motivated to buy different products and can therefore be very profitable.
Average price
How much does a certain product cost and how big is your profit margin on each of the product. All other things being equal you should focus on promoting on the products with the highest price, that give you the highest return per sale.