I always reccommend Sassie to new shoppers. They have a large amount of companies to sign up with and I really like the fact that you can use the same password for every account connected with them. I also reccommend that you open a brand new email account only for Secret Shopping. You may receive so many emails for shops daily and that way you can just delete all if you are not interested in shopping that day.
Go to: www.sassieshop.com - Click on Contact Us - Click on Shoppers. I think the rest is self explanatory. It will show you links of companies to apply with. Apply for all. All will be safe sites as well.
Also, NEVER pay to apply with any company!
Good Luck!