Choosing your niche is a somewhat personal choice. It pays to choose something that you are interested in, and fits with your abilities and interests. The type of site, marketing model and the complexity of the services you will build on this depend on how you fit with certain stereotypes.
If you are a writer, and expert/knowledgeable in a particular field, you might consider a blog
If you are a programmer, you might consider a software or service
If you are marketing focused, you might consider affiliate marketing
If you are business focused, you might consider opening an online store
Of course these are just stereotypes, everyone is unique, but it will help guide you. I would always emphasis that you should focus on areas that you are interested in, so you don’t lose interest in the project after a while.
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