I recommend multiple sources of income simply because it's possible to do as easily once you get the basics right.
To take it from the beginning, there are five ways to make money online. Selling your own product is the first. Selling someone else's product as an affiliate is the next. You could also market systems to train and develop people or use systems like stock-trading. Or you could engage in network marketing by recruiting networkers online. And finally you could do secretarial work such as outsourcing, surveys, transcription etc.
I do not recommend the last option as one under 'business' because it is more of a job. You work for someone on a fixed pay. You cannot scale up and use your talent and learning for exponential growth. Nevertheless it is listed for completeness.
Coming to the first two options which I delved into least, you would perhaps like to start with affiliate products. Why? No risk or capital investment in anything except yourself. You invest in learning the methods of online business and create the basis for multiple streams of business income online.
I was personally exploring the space of qigong, which is a method of utilizing life energy for healing and mediation. It is really very powerful. During this process I came across a gentleman who had 30 years of experience in this subject. Now I do not know qigong since 30 years but I know how to create a source of income online. So we partnered.
Now everything I learned I put up on my site and everything he knew he supplied to interested people who clicked on his ad. I made a commission on the sales he generated through his ad.
My work? To generate traffic to my site. This then becomes one site of many that I have learned to generate traffic for. More people come... more check out the information... more buy... more commissions! That's how the cycle goes.
What's more, through an AdSense account I also diverted some of Google's advertising to my site. This brought in another revenue stream on the same property - which incidentally exclusively belongs to me!
So the approach to multiple streams of business income online is much like one's approach to a department store. You build one or several and get occupants. Plus traffic. And you are home!
To learn how to start creating the traffic for your sites, read the author's post on creating multiple sources of business income. To watch a video which explains this qigong example check out http://www.youtube.com/watch?v=Jdpw9WGUeIk