How to file if My boss died, and I got a new job?

I'll assume you mean that you don't expect to be getting a W-2 from your dead boss's company. 

No W-2. Missing w-2:

If you haven’t received your W-2, follow these four steps:

1. Contact your employer (is there somebody associated with him that might still do that, wife, partner, family member, etc) to see if you will be getting one. 

2. Contact the IRS If you do not receive your W-2 by February 15th, contact the IRS for assistance at 800-829-1040. When you call, you must provide your name, address, city and state, including zip code, Social Security number, phone number and have the following information:

• Employer’s name, address, city and state, including zip code and phone number

• Dates of employment

• An estimate of the wages you earned, the federal income tax withheld, and when you worked for that employer during 2022. The estimate should be based on year-to-date information from your final pay stub or leave-and-earnings statement, if possible. 

3. File your return. You still must file your tax return even if you do not receive your Form W-2. If you have not received your Form W-2 by the due date, and have completed steps 1 and 2, you may use Form 4852, Substitute for Form W-2, Wage and Tax Statement. Attach Form 4852 to the return, estimating income and withholding taxes as accurately as possible. Your final pay stub is the best source of info. There may be a delay in any refund due while the information is verified. 

If you were expecting a form 1099-NEC (instead of a W-2), you can file a tax return, without it. 

Although it may be news to you that you “have a business” (and most of the other people who get their first 1099-NEC); The IRS considers anything on a form 1099-NEC to be self employment income. Enter the 1099 income, at 1099-MISC and Other Common income / income from a 1099-NEC . TurboTax (TT) will complete Schedule C  for you and allow you to deduct any expenses associated with this income. You'll also have to pay self employment tax (social security & Medicare) on any profits greater than $432. These types of payments are frequently done this way. Your "employer" is not the only one. If you try reporting the income as anything else, you chances of hearing from the IRS are  higher.

The IRS also considers undocumented cash income (no W-2 or 1099), for work performed, to be self employment income.