To promote a business online, I believe it is optimal to have a website no matter what your business is. However, if that is not in the budget, having a Facebook page - what is known as a Fanpage - can be a good start as Facebook has lots! of quality traffic.
If you develop a website, it needs to rank on the first page for keywords that are related to your business - what words would people use to search for your business?
The content on the page needs to be pertinent and can be optimized also to rank well in searches - that then take people to your website. There are many articles on the details of doing these tasks really well. If you need help - Fiverr is a great and affordable source to find people all around the world who can do these things for you.
Then you market it. How do you find the people who are looking for what you have to offer? ... where are they 'hanging out'? Social media - Facebook, Twitter, Instagram and Pintrest are four big ones that are currently being used to successfully promote a variety of businesses. Have a Facebook page is good - you can create posts that then can be turned into ads. You can create posts that are informative, knowledge based, and/or funny items that relate to your business. Essentially, how can you draw people to your FB page and keep them coming back? Again, there are people who can manage this for you if you do not have the time or want to learn how... Fiverr and Upwork can be good places to look... plus, there are more sources.
Searching blogs that are related to your business where people who might be needing your services/products are on asking questions or talking about their 'problems. There are people who can help with this and there are software programs that can find them quickly.
What kind of business you have will determine what is the best approach -- what would be the best methods to use.
PLUS .. always collect email addresses .. develop your system to collect them. Then you can email them consistently with information - adding value to them and offers that your business has. In the end, you are looking to get people to know, like and trust you - develop a relationship with them.