icecoldstarr,
Sign up with a new company or two every day because the first ones you sign up for might not be in your hometown.
When you accept jobs, read the instructions all the way through first. Sometimes, they are harder than they look, and it is easy to miss instructions. Don't just read the instructions, but read the questions you will be answering after the assignment.
If you are not getting jobs, keep signing up with new companies.
Then, if you start getting many offers, accept only a few at first. If you have too many assignments, it is easy to get confused. Work only with those companies that are easy and treat you right. They won't always have jobs in your area. Some months you might not have much.
Also try easy merchandising jobs. Some of the mystery shopping companies have merchandising, too. These jobs don't have as many questions, you don't have to hide why you are in the store, and the staff at both the merchandising company and at the store sometimes help you with the questions that confuse you.
After you get used to shopping and know what is expected of you, then you can look for more work.
You mentioned being smart. I don't know how far you went in school, but GED classes are usually free. GED classes in English would help a mystery shopper.