Initially, mystery shopping can be intimidating. While there's no reason to be afraid, being conscientious is a positive. Choose a shop that's interesting to you, and then read the guidelines and shop report. Then, only accept the assignment if you feel that you can meet and report all requirements.
Maybe a drive-thru at a fast food location... some grocery stores have minimal timings...office supply stores can be short and straightforward. After choosing one, before applying and being assigned, perhaps go for a 'practice run' at a similar shop. As an example, if you're considering McDonald's, go to Burger King and do a trial run following the shop requirements for McDonald's. (Burger King may think you're acting strangely, but so what! Then, if you actually accept a McDonald's in the future, you won't feel recognizeable.) When you get home, look at the McDonald's report, see if you met all requirements. It's kind of like testing yourself.
Welcome to mystery shopping and the forum. Make sure to read the forum section here for New Mystery Shoppers. You'll find helpful advice, including information on timing devices and other gadgets which may be useful.