You can find the calculator under other tax situations on the left hand side of your TurboTax screen.
Go to Form W4 estimated taxes. Follow the prompts. You can print the finished document and give to your employer.
You can also always update your W4 at work. Here are 5 reasons you should adjust your W4 withholdings.
5-reasons-to-adjust-your-w-4-withholding Step 2 of W4 section C check the box for multiple jobs.
Here is the link to the W4 calculator W4 calculator
If you have not yet filed your taxes you can go back in and get access to the W4. If the return has already been efiled it woud say already submitted.
I hope i was able to answer your questions. Have a wonderful day!