Well, before you get started make sure you have the zip file Usgard v1.1 for programmers. You WILL need an understanding of basic algebra, and number bases. Decimal, of course, is the number system we all use in our everyday lives. The number base for hex is 16.
Registers are high-speed memory locations in the processor itself.
It is better not to think of the registers as variables though. You will see why soon. The A register is special. It is sometimes called the accumulator. Often you can use the A register when others can't be used.