I would take a look at the ROBLOX Wiki to figure this out. I can't post the link to the specific article since I currently have no access to the website.
wiki.roblox.com
Steps to making a GUI Button.
Step 1: Go to the Basic Objects Window -- If you don't have it go to Insert>Basic Objects
Step 2: Select the Starter Gui Tab in Explorer
Step 3: When It is selected, go to your basic objects window and Click on "ScreenGui"
Step 4: Now Insert a "Frame" -- It is also in the basic objects window
step 5: Select the Frame and go to "Properties", when you are there you can change its Size,Position, Color,background color, Border Color,Border Size and rotation
--.You can change the frame to anything you want it to look like.--
Step 6: Now Insert a "TextButton" You can now change this text button however you like, or even put scripts in it.