I'm trying to make a program to simulate a parabolic movement, so i've added 2 entries, one for the initial velocity and other for the angle, anyway you probably don't care about that xD
what i want is to take the number entered in the entry and put it in a variable to do the math, and then show everything in a dialog or something.
how do i do that?

Depends on what language you're using. In C or C++ you can use the atof or sscanf functions to convert the string to a double value. Most scripting languages (Perl, Python) will automatically convert it for you. Other languages have other methods.

OH! sorry i forgot that, the language is actually C#. I really dont like c# that much (micro$oft) but thats my assignment so...

In that case, you should be able to just call GetText() on one entry to get what the user typed as a string, then use whatever conversion function C# provides to convert it to an int/double/whatever (sorry, not a C# programmer). Repeat with the other entry, and you have your two numbers to do the math with.

Showing the result is similar. If you just want distance traveled (for instance), just add a label to the dialog, and set its Text property to the answer. If you're trying to make a graph, that's a little harder. You'll have to use a DrawingArea for that.