Surely your teacher/lecturer does not expect you to re-invent the wheel, but to first see whether there is an established opinion about this.
e.g. you should first look for things like:
1. a textbook on Japanese linguistics.
2. scientific papers on Japanese linguistics or comparative linguistics.
You can then cite where you get facts from. It is always better to cite something like a book or paper than a webpage (as a webpage can change).
A quick search on books.google.com shows
An introduction to Japanese linguistics by Natsuko Tsujimura (preview only)
Infixes are bound morphemes that are inserted in the middle of a word rather than being placed before or after it. Japanese does not have any example of infixes.
A minute on
Google Scholar
turned up a paper called
Linguistic Distance: A Quantitative Measure of the Distance Between English and Other Languages
which is about comparing measures of similarity between languages to difficulty in learning, as measured both by the ability of American English speakers to learn a given language as L2, and the ability of immigrants to Canada to learn English (or French) depending on their native language.
There are surely many other sources. Your institution (I am assuming you are university level) will probably have access to many journals, so you should be able to find a few you have access to.