On line donation Thank You
There are a few places that contribute to the TY email.

On the Receipt tab of the Contribution page config you can add a message there in "Receipt Message".

Also any 'pay later' instructions will also be inserted.

The main template that sucks those bits in and adds a lot of other bits is under > Administer > Communications > Message Templates then click on System Workflow Messages and look for eg Contributions - Receipt (on-line)