You can create an affiliate website about any topic you like.
To improve your chances of success, find a theme that you are passionate about so your interest level will remain high.
Then narrow down your topic into a true niche, choose a smaller niche based on your theme. As an example, if your theme was tea, a smaller niche could be white tea.
See my article: How to choose your niche?
It’s important to research your niche to find out what keywords and phrases are being searched for, how much competition the niche has, and what kinds of affiliate-marketing opportunities it will be able to provide.
Research your theme:
Visit the free Google keyword tool and type your main keywords into the search box.
The keyword tool will give you information on how much competition your keyword has, the search volume and many other related keyword choices.
The goal is to find a topic and keywords that have a decent number of monthly searches, but not a lot of competition.
Decide what your niche is:
You don’t want a website that offers vacation packages, flea medication for cats, or exercise equipment. It will confuse the readers and will let them know you are not an expert in any of these fields so they won’t stay on your website for long.
You want to stick to one topic per website so that you become an authority on the subject.
See my article: What is the best niche for affiliate marketing site?