Grab your passport and contact the salon, where you can buy SIM card of the chosen operator.
From the suggested numbers, select the one that you like. It is desirable that it was easy to memorize.
The seller will fill in necessary documents and will give you a box with a SIM card. It usually already has the money and the start-up rate, which can be switched independently on more suitable options.
If you want something creative, then buy the "platinum", "silver" or "gold" room. The cost of such numbers much higher than normal. They differ in the selection of numbers and rates.