Board Thread:Roblox Q&A/@comment-98.242.97.162-20130609045020/@comment-14066068-20130731202355

First, you need a Screen Gui that will hold your gui. Make a Text Button inside the Screen Gui. Now, you need a Script, preferably inside the Text Button. In the script, you need a function that is connected to MouseButton1Click event of the Text Button. Now you need an Instance.new function, that will create your second gui, which will pop out when you click the Text Button. If you know all this, then the problem may be in Position and Size of the new gui. All new guis appear in top left of your screen, which is at 0 on X and 0 on Y axis. You can't see them because they don't have size, a.k.a their Size is 0. Now you should use UDim2.new function on the Position and Size property of the new Gui. Also, all new Instances appear in nil. So, you have to set their Parent property, preferably to the Text Button. All you need now is end that will end the function.

The best thing would be if you google "Roblox Gui". You will find plenty of tutorials. There are some that show you everything, from how to get into Roblox Studio to how to make awesome Guis. Find the one that suits you, and your knowledge the best.