Go through past exams and note down every single formula that they use. List them all on a page describing what it does, notation, and an example.
Edit: On further consideration and after reading posts below I see that this is probably better after you actually use the formulas in Excel. You'll spend many hours messing around with Excel formulas while doing SACs and such, the notes are probably for near the end of the year, to perfect the syntax, not before term starts. I kind of forgot that the syntax probably looks like gibberish if you haven't used them before
*Insert description here* Insert notes such as table must be sorted for VLOOKUP to work. I *think* you could put a zero somewhere and it won't need to be sorted, can't remember right now. VLOOKUP IS ESSENTIAL know how to use this perfectly
=VLOOKUP(lookup_value, table_array, col_index_num, [range_lookup]) (just get this from excel or some list somewhere)
=VLOOKUP(D2,D3:E4,2,TRUE)
Other essential formulas
Sum
Product
Other basic mathematical ones probably
Average
If
Countif
Count
Max
Min
But basically some of these such as =Sum can be replaced with other methods, such as manually adding up cells, but you don't want to do that.
lol can't think of any other useful ones right now, you'll learn them all during your Excel SAC anyway. I think it's important to actually know how to write the formulas perfectly; being able to use them Excel is much easier than being able to conjure up perfect syntax on paper without having Excel guiding you step by step.