If, for example you want to send a message to you window to bring it to the top of the z-order, add the following code into your class declaration, e.g.:
public class MyClass {
private bool x = false;
[DllImport("user32.dll",EntryPoint="BringWindowToTop")]
private static extern int BringWindowToTop(int hWnd);
The EntryPoint is not required - if omitted the entry point into the dll will be the name of the method.
To use the imported method, just call as normal, e.g.
BringWindowToTop(MyForm.Handle.ToInt32());