You will need to contact your employer to give you a corrected W2. If they will not send you one, then you can use your final pay stub to try to report your income, including tips and taxes withheld. You will do this by entering the information from your pay stub in the section for the W2. Then after you enter everything, the next screen will say, do any of these uncommon things apply to you? Check the box next to "Didn't get a W-2 (manually calculated my earnings) and then TurboTax will walk you through the steps of preparing a substitute W-2.
There are two scenarios -
The first is if you reported those tips to your employer and they did not enter those tips on the W-2 you received. If that is the case then see this TurboTax support FAQ or what you should do - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-forms/enter-unreported-tips/L0BGoGCZn_US_en_US?uid=lsme21yi
The other scenario is if you did not report the tips to your employer. If this is the case then after you have entered your W-2 you will land on a screen labeled - Let's check for uncommon situations
Check the box labeled Unreported tips
This will allow you to report the tips received, have the income entered on your tax return and also pay the Social Security and Medicare taxes on the income.
Reported using Form 4137, Social Security and Medicare Tax On Unreported Tip Income