What should I start with when making a game?

What should I start with when making a game?

I am going to make a game in 2D or 3D. But I have a problem I can't decide
if I should do the AI, physics, graphics or level designs first and if I
should use 2D or 3D.
If I use 2D I will use eclipse to program in Java. It will be a smaller
job and won't require highly detailed graphics, but it could be that
people won't like it as much as 3D. With 3D it is a bigger job, but I am
using unity to make my job easier. Unity is good, but I don't know if you
can stretch the limits as long as in programming. It is no way I will
program the whole 3D game.
And to the second question: What should I start with? AI, physics,
graphics, level designs? That would be good to know that too.
If I am going to make a 2D game I think I will make a platform game with
basic physics like gravity, and basic graphics. I don't think people will
be very impressed over that.
If I am going to make a 3D game I think it will be more like a big project
with a first person shooter camera and a big environment with high
detailed graphics. People would like that a little more
I can just a little bit about making textures to 3D objects. I have maked
textures to 2D objects so I have a little more experience about that. And
I can do much with programming with my experience, but it is limited of
course. 3D games will be very hard to work with.