This smells heavily of homework, so I won't give you the code!
However, it isn't difficult at all:
1) Handle the button Click event.
2) Convert the text box value to an integer, using the
int.Parse[
^] or
int.TryParse[
^] method - remember to report problems to the user! (I would use TryParse, so that I detect the error rather than getting an exception).
3) Compare the value with your bottom range: if it is greater than or equal to 1000, and less than or equal to 100000, then take 2% of the value.
4) If it isn't in the range above, try the next. Repeat for all ranges.
5) If you still haven't found it, report an error.
All you need to do is an IF statement with a TryParse, followed by an IF ... THEN... ELSE IF ... THEN ... ELSE ... construct.
Try it - it really isn't difficult!