Your return starts out with the standard deduction. It switches over to itemized deductions when you have enough that the total of all your itemized deductions exceeds the standard deduction.
In the case of medical expenses, you only receive a deduction for the amount of expenses OVER 10% of your adjusted gross income for the year (7.5% if age 65 or over). For example, if you have $3500 in expenses but your adjusted gross income is $30,000, you only have a $500 deduction, not enough to switch to itemized. If you have more itemized deductions, keep entering them to see if your total gets to be higher than your standard deduction.