Hello everyone! I'm working on my very first program ever, it's a calculator for (flightsim) pilots that calculates how fast you need to descend in order to reach your target at the altitude you want.

Here's what it looks like right now..


It works very well, but there are a few functions I want to add before I release it. I have managed to make it accept only numerous input and backspace, and if you write alphabetic letters, "Only numbers allowed" pops up.

BUT when I leave one or more text-boxes empty and press calculate, it crashes. What I need is a popup that says I have to write something in the input fields. How should I do that?

I also want to add some tooltips that pops up when I move the mouse cursor over the text fields. But again - how?

Would be very nice if some of you could help me, but keep in mind that I'm completely new to this and it's my first day of programming something. So please try to explain in an "easy" way...

Greets,
Even