The basic rule is that the fewer dependents you claim (S 0), the more that will be withheld from your paycheck.

For tax purposes, you will get a bigger paycheck if you select married, but you may end up owing tax at the end of the year.

There's a tool in Turbo Tax to figure out what you should claim on your W-4, provided you know approximately what 2018 will look like, income-wise.

To get there:  

1.  Click Federal Taxes on the left-hand side.

2.  Click Other Tax Situations along the top.

3.  Select Start or Revisit by Form W-4 and estimated taxes.

4.  Choose Yes to Do you want to change your W-4 withholdings for 2018?. 

5.  Walk through the next screens to enter your anticipated 2018 income.  When you successfully complete all the questions, you can print out a new W-4 to give to your HR department at work.

You haven't made any mistakes based on your question.  The important thing is to stay on top of things, which you're doing by asking this question.