It's going to be hard to help you without actually doing this for you, but here goes.
For the commission earned you will need to do various IF functions, nested within one another. An IF function takes the form:
=IF(<test>, <result if test is TRUE>, [<result if test is FALSE>])
This is equivalent to IF (test) THEN result if true ELSE result if false ENDIF (endif is simply indicated by the closing bracket)
You need to test if sales is >=4000, calculate based on 12.5%, else (test if >=1000, calc on 10%, else (test if >0, calc on 5%, else no result returned so you don't calculate it for people who have no sales, as per the spec))
this would be equivalent to
Null result returned
Because of the way the commision is structured, you don't need to test (>-1000 AND <4000) for example, because to get to the inner IF statements you have already excluded the other results.
To nest one function inside another, simply replace one of the result parts with the next condition, eg:
=IF(B2>=4000, B2*.125, IF(B2>=1000,B2*.1,<etc>... ) ) )
Personally I would use VLOOKUP against a lookup table rather than nested IFs for this as it would be much more flexible and extensible, but your project says to use IF statements (either because the person setting the course does not know better or because they want you to prove you can properly nest the IFs in the right order. I hope it's the latter)
Hope this helps.
If you try and write the actual functions out and post your ideas, I'm happy to help you fix any problems, but I would like to see you have put some thought and effort into this before simply "doing it for you" (unless you want to pay my usual consulting rates, of course