LÖVE

What genre?
Fantasy, Sci-fi, Apocalyptic, Post-apocalyptic, Romance, ... ?

What kind of game?
Platformer, MetroidVania, Isometric, Board-Game, Card Game,..?

I suggest study very old games (use emulators or YouTube) and think about these questions: what is the minimum amount of elements that can make a game a game. Example: pong -> 4 elements (field/screen, player1, player2, ball). What is the minimum amount of rules that can make a game a game. pong -> player1 must no let the ball pass and make it so that player2 cannot stop the ball. player2 must do the opposite. A single end game condition. pong -> a player reached a certain score. He wins, the other lose.

My guess is you don't know what do do partly because you could do anything. By forcing yourself to make a game out of the minimum of pieces and rules you will get ideas by trying to go around the limitations Also, don't think too hard about what limitations to choose. They can be arbitrary. Try a bunch of random ones! If you are still stuck, clone arcade classics (missile command, pong, breakout…). The more the better. Then try to randomly mix them and see what happens

To get ideas is very close to randomly mix things that you saw until you get something that seems interesting to you! So another advice would be take some time to randomly read, watch, play anything you can get your hands on. The more shapes and ideas you have in your head, the more you have to play with and mix to create new things.

Hope this helps, feel free to ask if something rings a bell but you need more details