Graduation speech quotes, help me!!?

I'm trying to write an amazing graduation speech for my graduation! I would LOVE to include some great quotes from well known poets, U.S. presidents, or people that are well known in our society (ex. Dr. Seuss, Walt Disney, Oprah, etc). If you can give me some great quotes from great people that would be perfect for a graduation speech, then I will be SO THANKFUL! Anything about inspiration, nature (about the stars and life), or anything that would fit into a graduation speech! I've already looked thru a lot of quote related websites, but I'm sure there are millions of quotes that I have not yet read. So, send me some of your faves! Anything will help me out! THANKS SO MUCH!!!!