How do I save the photos I took on iPhone on my iPad?
Presumably you are referring to My Photo Stream, which deletes photos after 30 days or 1000 images, which comes first. You don't say which version of iOS you are running, but the below will work on v8.

You can save photos to your device from My Photo Stream by tapping the Share icon (an upward pointing arrow) from any photo, then tapping Save Image. You can also select multiple photos first (tap Select at the top right of the album view, then tap on images to select them), again followed by tapping the Share icon then Save Images.