I currently have 33 shares of common stock at BNY Mellon from a previous employer. What do I need to do if I want to withdraw the shares or transfer into money?

When you say the shares are "at BNY Mellon" do you mean you have a stock account with BNY Mellon? If so, you can either sell the shares directly out of that account or you can establish a new brokerage account somewhere else (such as at Schwab, TD Waterhouse, ETrade, etc) and transfer your shares to the new account. Then you can sell the sares from there if you want. When you set up your new account you can submit paperwork to make the transfer of stock assets from BNY Mellon to the new account.