create an idea of the characters & world you want to play in, write a story, define your player character and get a thought WHY he/she is involved in that story and develop that story (and in best case also develop more than the weapon of the main char but himself

coward > hero!thumbs up). produce scetches of basic design (with paper & pencil, digital basic-design sucks every time

), of all levels,weapons, chars, landscapes, suroundings etc. in short terms: storyboard & linescript as made for movies and also often for books.
think about leveldesign, how to build suroundings to force the AIs to do things and to avoid bugs and chars running against walls. where would the enemy see the char and is there anything in straight between char & enemy? from where will he enter the room? use leveldesign to make it easy for the scripts to work, and you can use simple scripts and save memory.