Is it going to be player vs player or player vs computer?
In either case, you need to divide the playing field into a grid.
That's a start.
But be warned, when I wrote it in VB for player vs computer it turned out to be around 1200 lines of code and comments becasue I included a system in which the computer actually coordinates it's moves instead of having it take a rondom place.
Don't take that 1200 as a realistic number though. After I finished tic-tac-toe in VB, I somehow found a way to program Pong in VB using 1400 lines of code and a whole lot of timers!
Mmm...bloated code.
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