I've attached a series of screenshots that will hopefully help. in the below image I have a big red arrow pointing to the button that starts you on the solo game path, tap it to bring you to the next step.
In the below screen shot I've put an outline on the play button that will start a classic game. You can also see I have the other game modes unlocked and they say "2" with a coin symbol. Those are their start buttons but we'll skip them for now. This will take you to into our Classic game mode.
In the below image you can see what the game selection screen will look like if you leave a game in progress. There is "Resume" to continue the game you have started, or "Restart" if you want to delete that game and start over. When you tap Restart it will show a prompt asking you to confirm and if you say yes, it will change back to the above image with just a Play button.
In the next image you can see what the game set up will look like. I have put orange borders around the difficulty buttons, you can tap these to change the difficulty or even turn off one or two of the AI. You can tap the arrows next to the AI pictures to change the AI you are playing against. The bottom image shows what the game set up would look like if I turned off the third AI and made the first AI medium and the second AI hard difficulty.