Oh, I just love this essay! It is so typically Mark Twain. As you know, it is a satire. He has been asked to give a speech to some young girls, and so he pokes fun at the usual advice given to young people by adults. For example:
Always obey your parents, when they are present. This is the best policy in the long run, because if you don’t, they will make you. Most parents think they know better than you do, and you can generally make more by humoring that superstition than you can by acting on your own better judgment.
We cannot write your essay for you, but we can help you get started. Think about some advice that you have received from adults, your parents, your teachers, or some advice that is really more like a lecture, and then do the same thing that Twain does - make fun of it. You could even paraphrase his words and put them into more modern English: For example, Twain says:
I have a few things in my mind which I have often longed to say for the instruction of the young; for it is in one’s tender early years that such things will best take root and be most enduring and most valuable. First, then. I will say to you my young friends
You can say (let's use the venue of a commencement address, since you are a senior):
Yo, dudes! I have been asked to give you all some advice as your class valedictorian. Soon we are all going to leave this prison we call school. Yes, we will be out in the real world. So what advice do I have for you guys? I have learned a lot of good lessons during my four years of high school, so.........here goes.
First, if someone offends you, don't get revenge. At least not right away. Turn the other cheek. Temporarily, that is. While you turn that cheek, be planning your future attack. Maybe you can put a dead fish in the person's locker.
Second, get up early every day. Remember that the early bird gets the worm. Not that you really want that disgusting worm - what you really want is to beat your roomate to the shower. If you shower first, your roomate has to clean the shower because he will be the last one in it. Also, you get first dibs on what little milk is left in the frig for your cereal.
You get the idea. If you are rewriting Advice to Youth, it must be a satire, so it cannot really be serious. Have fun with it!