It's too late to do anything for 2018. It will all get straightened out when you file your 2018 tax return.
You should give your employer a new W-4 for 2019. After you get your first paycheck in 2019, you can use the Withholding Calculator on the IRS web site to figure out what to put on your W-4 form. The IRS Withholding Calculator is at the following link.
https://www.irs.gov/individuals/irs-withholding-calculator
