I'm gonna jump in here and say: Good Luck.

But I'll leave you with some pointers I've picked up over my education.

1) 8-bits is your friend, it was a standard for a reason.

2) NAND gates are also your friend, since RP, P:R and BP all have these gates already built on one tile for you and can be used to do ALL logical and mathematical functions electronically.

3) Start small. Start with basic addition of two 8-bit words (Two bytes).

4) Grow from there with basic multiplication, then devise a system to switch between the two.

5) Pray to a deity of your choice, do some reading of electronic subtraction/division and try to build that into your system.

You now should have a basic calculator.

I have a diagram somewhere of a generic CPU if you get that far