The 1040A is only available in the TurboTax Free Edition. It looks like you are currently posting from that edition. If not, go here Free Edition and the requirements for using that version are listed below.
The TurboTax software chooses the appropriate 1040 form that applies to your tax situation based on the information that you enter. So once you enter all tax information, the generated 1040 is the one you need to file your return. And if you need to itemize, the 1040A will not allow for itemized deductions; those will go to long form 1040. More details can be found here Which Tax Form Should I File – 1040EZ, 1040A, 1040
The filing status and exemptions section of Form 1040A is similar to the corresponding section on Form 1040. Due to the limited types of income you can receive and the limited adjustments to income you can make on Form 1040A, the income and adjusted gross income sections of Form 1040A are much shorter. One of the most significant differences between the two forms is that you can itemize deductions on Form 1040 but not on Form 1040A.