Good day sir I forget my Apple ID and there is iCloud lock. How can I use my iphone

If you forgot your Apple ID

https://support.apple.com/HT201354

If you forgot your Apple ID password - https://support.apple.com/HT201487

Edit: It appears from your second post you did not forget them. Enter them to unlock your phone.

Turn off Activation Lock --> https://support.apple.com/HT201441

you will need to contact the previous owner to have them log out of iCloud or go to iCloud.com/Find My Phone and erase the device, then remove it from their account. If you can’t do that, the device is useless. 

Activation Lock - Turn off Find My iPhone