How to make a game faster?

I am new on Unity.

I have added a tree in my game and when i would to run it. The game is playing very slow. My game is ampty. i don't have any gameobjects. i have just a fps controller with weapons and when i want to add only one tree. The game is running very slow. I have tried to build my game and run it. but it's the same story, but a little bit faster. That's not enough. I have seen some games, it's full with gameobjects but the game is still fast.

Can somebody help me with this problem.

Sorry for my english. I'm from Holland.

Did you model this tree yourself?? if so check how many polygons it has (all those little squares on your models (for some triangles)) unity won't import the model if the model has over 6000 poly's but if you had like 5999 then it will import, but go very slow ;) Hope that's clear enough for everyone

I have seen this question before i asked my own question. I have done all of the step, that were told to do. but it doesn't working. I was looking for my graphic card, maybe that was the problem. But nothing. This computer is new, with a very good graphic card. one of the newest Intel i5 graphic card. Maybe someone can help me?

My pc can read 3D. I have bought a computer specially for games. I tried something, maybe it will help, and yes it helps a little bit. I returned all of the GameObjects from bumped specular to diffuse. I can see some results. But when i place three trees. The game will run slow again.

I'm new on unity, so how can i make sure of the polygon count is not enormous?

Are there someone else suggestions to fix up the problem?