Try not to select what is important. Rather, treat every aspect of observed behavior as potentially important and thus worthy of being recorded. Document your own behavior as well. Do not worry about stylistic niceties in writing up your fieldnotes. They should be typed, but grammar and punctuation are not the point here.
Observational detail is the point.
Write quickly and very soon after the observations, while the details are still present in sense memory. You should produce several pages of notes. Five pages of close description of ten minutes worth of interaction is better than five pages of vague descriptions of what went on for several hours. Before conducting the interview, prepare a notification and consent form for the interviewee. The form should inform the interviewee of the purpose of the interview and any risks or benefits connected with participating in the interview and ask for formal permission to proceed.