If you're serious about making money online, start a blog. Blogging is one of the easiest and most sustainable income sources. As long as the blog is setup the right way, in the right niche, with the right content targeted at the right audience, and the offer is complementary to the content, you could make a tremendous amount of passive income from a blog.
If you’re a fitness buff and have the right combination of charisma and business sense, working as a part-time online personal trainer can be both physically and financially rewarding. Once you build up a reputation and client base for yourself, it could easily turn into a full-time endeavor for you. Check out this interview with several fitness blog owners who are making a living online, from MonetizePros. As well, I'd recommend checking out this resource if you want to take this business idea seriously and get started with a business plan for your personal fitness trainer business today.
Sponsored posts work much in the same way as paid guest posts, but they are posted by big businesses instead of individual bloggers. Therefore, the scope for fees is much higher, as businesses have larger marketing budgets than humble bloggers. Having sponsored posts by large companies will also help promote your site as reputable and as a leader in its field.
If you’re looking for inspiration, my friend Michelle Schroeder-Gardner of the website Making Sense of Sense has become the expert on all things affiliate marketing. Michelle earns more than $100,000 per month from her blog and the bulk of her income comes from affiliate sales. Michelle has had so much success with affiliate marketing that she even has her own course called Making Sense of Affiliate Marketing.