1- How can I define board shape? especially odd shaped boards like importing autocad drawing or some free software output like autocad.
Export as a DXF file in the R12 format, then import to the edge cuts layer
2- Is there any comprehensive library which contains footprints?
The built in library will cover about 85% of things, specific sized transformers, and a few brand specific footprints may not be there
3- I want to increase copper under some power mosfets or resistors? is that possible and how?
Draw a "filled zone" in the shape you desire, set it to the net name of the pad you want it to add to, then select solid or thermal connection depending on how you want it (thermal is for easier soldering)
4- How about adding thermal vias to it?
draw trace from the center of the pad, turn off the snapping, and place vias where you want them. You may also want to turn off auto delete tracks if it fights you.
5-Have yet to do it, so not to sure
6- How about 3d view? do I need extra stuff? how to access it?
Top of the screen, veiw / 3D veiwer, for the built in libraries the models should be there, anything you make would either need one assigned or added
7- I also want to drill some holes... how is it done?
If you want holes or shapes in your PCB, draw graphical objects on the "edge cuts" layer, e.g. a circle would be a mounting hole.
If you want single or an array of plated holes, they are available in the footprints library. this can be accessed by the 4th icon on the right hand toolbar, looks like a 6 pin IC.