While I was doing WordPress speed optimization I noticed lots of people needed it, but very few people supplied it (there were a lack of services and tutorials when I researched Google). I also knew hosting was the #1 factor of website speed factor and these companies paid up to $200/sale. Hosting is a competitive space but the commissions and lack of supply enticed me.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you. 
Be honest. Talk about what you like and don’t like. Be fair and build trust. It will serve you well later. For examples, check out my review posts about Elite Blog Academy and Self Publishing 101. I get emails frequently from people who tell me they decided to purchase one of those courses through my affiliate link because it was the most balanced review they found.

If you have to earn $50-$100 commission you have to sell product of cost $500-$1000 (If you’ll get 10% commission). If anyone who is reading have the potential to sell product of this much amount then why not he/she should start selling his/her own product. And money ultimately comes when a service or product is sold! So if you can write an Ebook then write another which you can sell for money!
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.
×