What I can do to reset my iPhone?

So the real story emerges. You can't use this phone, unless your cousin removes it from his account. If he can't do that or is unwilling to do that, the phone is 100% useless to you. We can't help you. Apple WON'T help you.

Either get your cousin to remove the phone from his account, so it can be erased for you to set it up or recycle the phone responsibly as it's a useless brick to you.

Give your cousin this link --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

He would need to follow the directions on If You Don't Have Your Phone.

And if by chance your cousin didn't really give you this phone, read this --> The All Too Common SAD Reality of Buying … - Apple Community

You're Apple ID is your email. It's pretty hard to forget one's email. Give us more details about the phone, like if you are the original buyer, etc.