This summer, my band teacher let me borrow a melophone to practice for marching band. The only thing is that I don't know how to tune the valves!!! please help

Ah, good question. I wish more people asked this question more often.
Here is a basic method to tune your mellophone.

Step One - play for a few minutes. While you are playing get a feel for how the notes feel when you are playing them. (I say this because 90% of people play their tuning notes differently than when they play normally resulting in tuning that doesn't tune).

Step Two - tune the main tuning slide on a common open note. If you are going to be playing a lot of high notes use C 3rd space treble clef. If you are going to be playing a lot of lower notes, use G 2nd line treble clef.

Step Three - tune the 2nd valve slide by playing B or F#. When in tune mark it with a pencil.

Step Four - tune the 1st valve slide by playing a Bb, or F. When in tune mark it with a pencil.

Step Five - tune your 1/2 valve combo by playing A or E. When tuning this combo you will move the 1st slide in twice as far as the 2nd valve slide. So if you are flat move the 2nd valve in 1/16th of an inch and the 1st valve in 1/8th of an inch. Continue until you are in tune. When in tune mark them with a pencil. Now move your slides so that each slide is between the two pencil marks you made.

Step Six - tune your 2/3 valve combo by playing Ab, or Eb. Do not move your 2nd valve slide, only move your 3rd valve slide until you are in tune.

All Done.