I have forgotten my password for my icloud.com account that i used to first activate the phone. how do i retrieve the password

I have an iphone 5, operating on iSO 6.0.3. It had switch off but managed to turn it on. Its asking for activation. I have followed all the steps up to were am now suppose to enter eg a....@icloud.com.

The challenge I have is that I have forgotten the password of this account which I used when I activated the phone first time that was way by in 2013. How do I retrieve the password or what can i do to activate my phone am stuck.