say you decided to build a calculator :
1- you will need to build an adder, multiplier, subtractor, divisor .. etc.
2- you will need to run one of these operations once one enables it (by pushing the button).
3- you will need to have couple of registers to save the operands of any operation and the results.
4- you will need to write a 7-segment or LCD driver.
i guess this can be the simpliest start in HDL. Try to look at an existing calculator and take it as a reference .. don't implement the fancy stuff.