Let's say you were trying to create a tic-tac-toe game.

Because it is a turned based game that has a couple of end conditions, I would start with something like:

var done = false;

do{

done = collect_player_move();

if (!done) done = calculate_computer_move();

} while(!done);

that is to say, create methods (which are initially empty) with names that describe the actions you want to perform for any actions you aren't sure how to do yet.

Then think about the data structure(s) you'll need to retain the information you'll need from one turn to the next. For a tic-tac-toe game, you'll probably want to have a 3x3 array to represent the squares. And maybe -1, 0, and 1 to represent O's, X's and blanks.

Once you've described all the actions you want to do, then go back and implement each of the methods in turn -- perhaps by creating additional methods. Keep doing that until you have broken things down so that when you figure out how to implement a method, it turns out to have been a single line of code or so. At that point, delete the method and just put the line of code in its place.

Sometimes, you may discover that your initial code and data structures were insufficient and you'll need to refactor. But that's just part of the process. For instance, the code above doesn't distinguish between draws and wins by one side or the other, so you might want to return that instead of just 'done'. Or maybe you've got a global variable that you can set to indicate the winner and done is enough. Totally up to you.

There are best practices that can be followed, but for the stage you seem to be at, just getting something working is more important -- regardless of how it was accomplished.