Please help and explain how to work 1A and 1B because I don't relaly understand it. Thank you!

Quartiles are basically 25% markers, i.e. the median is the 2nd quartile as it represents the 50% marker.

Therefore the 3rd quartile is 75% - with 100 students, that means 75 students send less than 102 messages, and therefore approximately 25 students in the sample send more than 102 messages.
If you draw a frequency graph, you can draw the quartile markers in as vertical lines to divide up the data.