Like how does it work? For example, if the commission is a traditional art, do they send the whole original work? And please give me an idea about what its like?
It depends on the artist and what you agree upon when the artist takes the commission. Some will ship you the piece, and the cost of shipping is added on to the initial cost. Some might have an option later where, once the piece is complete, you can decide if you want it shipped to you or not. I think typically you get emailed a high-res scan of the finished artwork.
The commission process varies from artist to artist. If you have a certain artist in mind, check out their website or account, and see if they have any sort of TOS (terms of service) which should explain their rules in relation to the commission process. Then simply contact the artist and ask if they are open for commissions. As Cnids said, it's fairly standard for the artist to take some sort of downpayment for the commission, so if you back out after they've already started, they don't get screwed.
Most artists have different commission setups. It's probably best to talk to the artist you're thinking of commissioning (or read their info sheet) to understand what you'll get in return.
A fairly standard setup for commissions is to request an up-front payment (I take half because I don't want to get screwed; some people ask for less or even 100%) and then the rest at the piece's finish.
This depends on what you agree on with the artist. Shipping for originals is usually not included in the price and it's up to you if you want to pay for the original or not. You find the artist you can afford and send a commission request. The rest of the process is up to them, but usually includes some upfront payment on your part, and sketches and WIP shots from them along the way.
Thanks xD