I couldn't agree more!

"I think pizza is the best food on earth!"

"Me, too!"
"So do I!"
"I do, too!"

All three of these forms can be used in informal English to agree with a positive statement another person has just made.

Of the three, "Me, too." is the easiest to use, as you don't have to worry about the auxiliary verb.

For "So ___ I." and "I ___, too." you need to use the appropriate auxiliary: am/is/are, can/could, do/does/did, have/has/had, should, will, would, etc. Notice that we do NOT use a contract of the auxiliary in the answer.

For example:

"I'm a big U2 fan."
"So am I." / "I am, too."

"I've been working really hard lately."
"So have I." / "I have, too."

"I should really do an intensive English course."
"So should I." / "I should, too."

"I went to the cinema [AmEng the movies] on Saturday"
"So did I." / "I did, too."

You'll notice than in this last example, because there is no auxiliary verb in the original statement, you need to use "do" as your auxiliary, as in the pizza example above. Because the original statement was in the past, we use "did".

These forms can also be used for other people than yourself. Let's adapt the last example:

"We went to the cinema [AmEng the movies] on Saturday"
"So did we." / "We did, too."

"I want to go to the cinema [AmEng the movies] on Saturday"
"So does Mary." / "Mary does, too."