i buy the iphone from someone, i was using it normal but i factory reset it now is asking the first person icloud

You will need to contact the original owner and have them remove the phone from their account. You can give them this guidance:

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

If you are unable to contact them or they are unable or unwilling to remove the phone, then the phone is useless to you. Please recycle it responsibly.

activation lock is a theft detterent system, one of the best out there.

you will have to ask the previous owner to remove the activation lock

see this link for reference

How to remove Activation Lock - Apple Support