Registration Mail

Hi.
Can please help me with the following mails…

After registration the users see this…
You have successfully created your account! To begin using this site you will need to activate your account via the email we have just sent to your address.
(I want to change it)

And after Account is Activated…
Your account was activated successfully! You can now log in with the username and password you provided when you signed up.
(I want to change this too)

kindly let me know where do I go to change these mails? I am a newbie to these things.

Awaiting your kind response.
Thank you.