Need Help with analysing a big data set

Hey All. So I have been asked to analysis a big set of data at work whilst in my internship. I really am unsure about where to even start but would like to try and have a go myself before asking for help. I would try and search for it myself but don't even know how to do that yet!

So, I have data from blood pressure readings. First of all, I need to group all the data in certain ranges. My column headers are 'id' 'year' 'month' 'day' and 'daily_SYS_bp_average'. 

So I need to check the earliest reading taken in Feb 2019. Is there a way to say year is 2019, month is 2, day is minimum and then sort the daily_SYS_bp_average within ranges? I then need to group the data according to those ranges to then do analysis on it. I will need to check the monthly average for each user each month and see how it has changed vs the original reading.

Sorry if this doesn't make sense, happy to clarify anything and thank you for any help!