How to decide the size of artboard for printing?

I'm not sure why you're not asking all these questions directly to your printer - how are you going to design something if you don't know the size they are going to print it?

Hi,
use the original size (100 %) inkluding the bleed for your artboard.
you could change the size with "command"+""alt"+"P"
My best.

Crucially, you need to forget about working in pixels. You get a size in inches or mm, and that is what you use. Don't slip into thinking in pixels, that's not relevant. 

I'm not familiar of using Illustrator for printing. All I know is that the resolution should be high (300dps)

Resolution when setting up a new Illustrator document is only relevant to raster effects (drop shadow etc.). It has no relevance to the overall PPI of the output file.

For example, I'm asked to deisng for a banner print that'll be printed on a stand, what are my steps to know how to size the artboard?

Should I ask them for the dimensions of the banner?

Yes.

and after i get that dimension, how much should the artboard size be? 

It should be whatever size they just told you.

and what about the bleed?

There are standard assumptions/rules-of-thumb, but it's best to ask the printer that.