You'll do fine. I don't know what you know so I will just say, make sure you measure to the length he wants them and then past that another inch or so for the seam while he is wearing them. You should pin things in place so it's easier and remember you can pull it out and do it again if you sucked it up (but you can't recut). Also, remember it's being folded under so no one will see it. It's a good thing to practice on. If they are jeans you're gonna need a bigger needle for your machine. If they're just pants you should do fine with a normal needle. Good luck!! I hope you love sewing--it's my fave.
My advice for projects is just do it! You'll never get good at anything if you don't just try. And like Amanda said, you can always unpick it if you mess up! You totally got this!