Right now, I am learning Lite-C. To learn, I am creating a simple pong game. But what should I do after this?

Well this is the idea of game development. Start using your imagination...

I know :P But I mean like, should I attempt a simple 1 leveled 3d game, or a different game like pong, or tetris, or a demo for a game, like maybe its just a 3d level, and the npc's chase you and you have to run, etc.....?

Well there is'nt anything that you must study after you have learned something.
If you are learning by yourself make your own mind what YOU want to learn next.

The most awarding is when you have a game idea that is not too much for your skill level and make it so it is playable. Make a little project. If you want to learn how to change levels, do seperate levels. If you want good visual appearance start to learn materials and shaders.

Trying to do different types of games helps also. Maybe at first try a sideways platform game. So you don't have to worry about the camera movement that much. Next time try a simple maze etc. Then move on to Third Person shooter...

Try using your own mind and manual as much as possible. It helps alot in the future!

I ,myself, am not far with GS3D, I'm learning Lite-C bit by bit - the basics, so I could even start doing something. I already have a basic idea I'd like to create for my first 3d game, so I'm working for it.

Make yourself motivated.