what you do is - you identify all the bits of data you need to store, and then design a set of tables in which to store them. some of the bits of data logically belong together in the same table
now doing this correctly needs some experience but the aim is to get to what is called a normalised structure - basically one where the same bit of data is never stored in more than place.
ignore any calculated figures, as they will be calculated from other data - eg you mention "termly average". ignore this. once you store the students, the modules they study, and the marks they gain in those modules, then you can calculate any statistic, such as the number of modules they studied, their total mark, their average mark, how they rank against other students etc etc - all from the base data.
there is often some stepwise refinement in this process. your initial attempt to construct the tables might not be 100% the best. so we try and do this first, and then as we start developing we realise we need to amend the table structure. the earlier in the process we can do this, achieve the optimal data structure, the easier and more harmonious the whole development becomes.
good luck
