Welcome to the forum Fani! Most of the information you need to gain an understanding about being a mystery shopper is in the threads pinned to the top of the New Mystery Shopper section. Kick off your shoes, grab a cup of coffee and settle in to do some reading.
To your specifics. Some companies pay by PayPal, some pay by direct deposit and some pay by check mailed to you. There are a few others that pay by more obscure methods, but you will see information about their payments as you attempt to register with them and may decide whether it is worth the bother or not. Each company has their own accounts payable schedule and some pay in a couple of weeks, some pay during the month following the month when the job was performed, some pay in two months. As you register with companies do note what they indicate their pay cycle is.