First is fine
Second is incorrect, but only due to the fact that it would be "My friend and I".
The third would be "are" and not "is", due to the fact that two is more than one and therefore is plural, and plurals need "are".
The fourth is correct, however it's missing punctuation. "We can't drive on: all the tires are flat" or "we cant drive on, all the tires are flat."
The colon (:) is used for adding information to explain the previous point in more depth, whereas a comma (,) is used for to create a pause in a sentence.