If your employer has given you the permissions to clock in from the mobile app, you must first ensure you are in Employee Mode. If you're not sure if you are in manager or employee mode, check out the article How do I switch between Manager Mode & Employee Mode on the App?
Once in employee mode, go to the clocking page and click on the large Start Work button to begin your shift.
When your shift is over, click on Finish Work.
This will allow you to rate your shift or provide any comments before you officially Clock Out.