I started working on May 17, 2018 and on my schedule A standard deduction, I got a $12,000 for being single. Is that correct? Should that number be prorated?

[VolvoGirl]

That's right.  Everyone gets the full Standard Deduction unless their Itemized Deductions on schedule A is more.  If you get the Standard Deduction you do not have a schedule A.  It's either one.

Are you doing a 2018 return?  The Online version is only for 2019 now.

For 2018 the standard deduction amounts are:

Single 12,000 + 1,600 for 65 and over or blind

HOH 18,000 + 1,600 for 65 and over or blind

Joint 24,000+ 1,300 for each 65 and over or blind

Married filing Separate 12,000 + 1,300 for 65 and over or blind

[Hal_Al]

The term "schedule A standard deduction" is a misnomer.   You either get the standard deduction OR you itemize deductions on Schedule A.  It's unlikely that someone who just started working would itemize.

So, yes, you get the full $12,000 standard deduction, even if you only worked part of the year.  It is not prorated. 

The only time the standard deduction would be less than $12,000  is if you are claimed as a dependent by someone else, e.g. your parents.