How can I lock cells so that the info can't be changed. (So that they can't
even try if possible)

Also is there a way that if I have

A1 and E3
and the user needs to input data into A1 before they can input data into E3

I want to be able to lock E3 until A1 has info then Unlock E3

But if the user deletes the data in A1 then the data in E3 is also deleted.

How can this be done?