Hi ,
if i understand you correctly, When workflow runs you need to look for a value of a field , If it matches your condition then you need to set value to other filed.
Here is the example.
Start creating a workflow
Add Check Condition, Here i am showing , If Zip code of account contains data , I am setting Phone through workflow.

Then Select step , Update Record. This is how it looks

Click on Set Properties And Update phone field.
You are good to go now.
Please mark it as answered if you are happy with answer.
Thanks,
Sreeni Pavalla