Should I talk about myself while writing a letter of recommendation for a student? First time writing a letter of recomendation. Never saw one about me. In this case I'm writing a letter for a PhD fellowship for an applicant who has not been my student.

I know this student personally, but we worked together only while writing the proposal.

As the title says, in this particular circumstance (if it matters), should the letter focus entirely on the student, or should I also talk about my research interests and how the student fits within them?
Only write about yourself if it clarifies why your opinion would be specifically relevant to judge the student. 

The reference is about the student, not yourself. Where more information about yourself is required to understand what you wish to say about the student, provide it. Otherwise, leave it out. The recipient will have to make up their mind as to whether you are (for them) a suitable referee or not.