I am very much tired working on Autocad 2010 for 3D modelling.
I don't know how to reduce the file size.
Can anyone help me out?
 

How large is the filesize right now?
I would start by running PURGE and then AUDIT.
Delete layer filters and layer states.
 

You might also want to think about whether you can break the drawing up into pieces and then Xref them into your main drawing.
 
Blocks are also a good way to reduce file size. If you have a lot of repetitive features, make blocks.

file size is small. already purged and audited. ill check the layer filters, but i try to use the least amount of layers that makes sense for the project.