There is an employer and an employee contribution. They're determined by weekly or monthly earnings.
For example, if your employee earns $30 per day x 5 = $150, the employee contribution is $3.15, the employer contribution is $9.65.
Some employers simply pay both parts to keep things easier.
The amounts are determined by pay ranges. Dept. of Social Security will provide you the schedules, pay periods, forms and all the help you need. In fact, I have found them to be the most professional of any GOB agency I have dealt with.