How do I create a conditional field?

What are you asking? How to apply these rules or what is the best way to apply the above conditions?

You are talking about dynamic form rules.

https://www.google.com/search?q=quickbase+dynamic+form+rules&oq=quickbase+dynamic+form+rules&...

You also should know that these are very straightforward to set up in forms, but there's a BIG loophole in QuickBase around the fact that grid edit or API calls will not have access to those rules, so users could circumvent your business logic as stated above. The form entry can lock them down to what you want, but not grid edit. This is the single biggest thing I wish QB would address, as table validation rules should be independent from forms and tied to the table setup more directly.

Create a formula field checkbox 

Condition should be like eg: if(condition, 1,0)

Then go to forms- click on dynamic rules - Create a new rule

If Formula Checkbox is checked then create the actions like this , make the field as required[what ever fields that you need to make it as required] , also automatically reverse action will be created by quick base itself , you can see on Otherwise Heading below actions on corresponding rule.