if i forget the ID and password how can i open my device ?

You will need to recover the id and password. Following is help for that: https://iforgot.apple.com/appleid

You can't. Both are required for access. Find My iPhone Activation Lock. If you are the original owner and have written proof of purchase then you could ask for help at the Apple Store.

You could have someone call you to get quick access. However, after that I do not know.