What should I do if I forgot my MacBook log in password?

if you are using the most recent OS (Catalina) you can reset it by following this;

https://support.apple.com/en-us/HT202860

If you are using Mojave or earlier follow these instructions;

https://support.apple.com/en-us/HT211672

Intel Computer

1 - Restart computer and immediately hold Command + r keys

2 - Top Tool Bar >> Utilities >> Terminal

3 - Type resetpassword as one word and no spaces

4 - Press Return key

5 - Follow the prompts and remember the New Password

M1 Computer 

1 - Choose Apple menu > > Shut Down.

2 - Press and hold the power button on your Mac until you see “Loading startup options.”

3 - Click Options, then click Continue. 

4 - Top Tool Bar >> Utilities >> Terminal

5 - Type resetpassword as one word and no spaces

6 - Press Return key

7 - Follow the prompts and remember the New Password