First, design your screens: 

1. Welcome Screen with a Start button
2. Game Screen, where you can add 9 buttons, in 3 rows, having each 3 columns; maybe a cancel button, if the user wants to resign while playing
3. End Game Screen, where you will show the result of the game and an option to play again.

The screens above are just an example, you could choose any other design structure you find appropriate.

Then, add some basic functionality blocks. For example, using the screens mentioned above, you should code the button in the first screen to move to second screen. Then, create some variables to remember various information: who is starting (player or computer).

Here you have an example project.