I work part time for a company that has a 401k for me. I am still working for them. I will be 72 in April of this year. When and how much must I withdraw from the 401k?

Contact your 401(k) institution or administrator. They usually send you the Required Minimum Distribution (RMD) automatically or you may have to request it. 

Please see the following: IRS Retirement Plan and IRA Required Minimum Distributions FAQs.

You may qualify for an exception from taking RMDs from your current employer-sponsored retirement account, such as a 401(k), 403(b), or small-business account, if:
- You're still working
- You do NOT own more than 5% of the business you work for
- You have an employer-sponsored retirement account with the business you work for.
If you meet all the criteria above, you may delay taking an RMD from the account until April 1 of the year after you retire. Keep in mind that this does not apply to IRAs or other accounts you may hold with companies you no longer work for.