You need the variable, Prompt, and MessageBoxOK events.
For fishing it'd look something like this:
onActivate > Player > VCE_modVariable > fish1 [Add] 1
onActivate > Client > centerPrint > You caught a fish! | 1
And then for selling the fish:
| is another text input box.
onActivate > Player > VCE_ifVariable > fish1 >= 1 | 1 2
onVariableFalse > Client > MessageBoxOK > Error | You do not have any fish.
onVariableTrue > Client > Prompt > Yes/No Would you like to sell your fish?
onPromptAccept > Player > VCE_modVariable > fish1 multiply 2
onPromptAccept > Player > VCE_modVariable > cash1 add <var:player:fish1>
onPromptAccept > Player > VCE_modVariable > fish1 set 0
onPromptAccept > Client > centerPrint > You now have $<var:player:cash1> | 3
3000 onPromptAccept > Client > MessageBoxOK > Thank you! | Thank you, come again!
If you need any events, any help, etc. just PM me.