Linking GUI input with script

Sorry Im new to Unity and I need help urgently!! I have a problem when it comes to integrating the GUI functions and scripts together. For example, I have script in C# that is used to calculate the speed of and object using:

Speed = Distance / Time;

The result is then displayed on the interface. I need to get users to input the distance through a GUI text box. How do I get the input and link it up with the equation I have in the script to produce a new answer?

I would appreciate it if anyone could help. Thanks!!