Just like everything else in real life, we usually have to start things by taking the first step. If we can never quite get there we usually will just forget about it. In other cases, though it will remain somewhere in our mind, and someday we may actually take action.
The thing is in the case of starting a business that you are literally wasting time where you could be earning extra money. You would maybe be surprised how simple that would be to do with just a little bit of effort. We do not have to immediately be tied-down but can just take the preliminary steps and then whenever we can we can make some progress.
Your business can be as simple as joining an affiliate program or two and investing just a little time to promote it in a casual way; for example to discuss the program in a blog post and include the affiliate link. Try to do that periodically. At the very least if you do this frequently enough the search engines may notice and index your site. This can result in ranking even with your smallest effort.
By the time you decide to get serious and do other things to promote your affiliate program, people may be able to find your link when they query your keywords. No telling if you could actually make sales at this level although it usually takes a bit more effort on your part.
Your business can also be something other than just having an affiliate program. However that may require a domain name, hosting account (both extremely inexpensive – lunch), and a product or service which can be whatever you want it to be. It can also be something you represent for someone else such as a product or service they have and would pay you for referrals (such as for instance health and beauty products).
There is very little investment if any beyond the technical things you need to have a website of your own. If you are planning on developing your own product or service someday, you can develop and/or change your website as many times as you change your mind on what you really want to do. Obviously it will take a more advanced skill set to invent something of your own.