java msgbox event

Could someone show me how to...

a) fire off an event that would run some java code.

b) Show me some java that would display a messgae box in ASP like the old VB 6 msgbox did.

Or, point me in the right direction to find out these things.

Thanks

If you don't know Javascript have a look at this control, it can be used as a message box or Input box and it's free to download and use and is very useful if you need to return values too.