i'm a first year student and i have to write a program which calculates the net profit(total after tax),given the gross profit .assuming that the tax is 20% of the gross profit .i must use pseudo code for this program.i don't know where to start with my program

Well, personally I'd start by asking my teacher for clarification if I was totally lost.
That's what they're there for.

Since I don't know what your teacher wants or expects, it's hard to help too much without a lot more details on the assignment.