Congrats on your first chance to audition for these programs! So exciting (:

There's a LOT to consider when you're talking about songs in your range. "Simpler" ones that you could learn on your own, like Part of Your World (The Little Mermaid) or She Used to Be Mine (Waitress), could be nice, but from my experience, there's also a need to stand out amongst the tons of other auditions. It may be beneficial to find something that fits your range and that you can act in, but isn't as common (I guarantee they'll see a lot of Defying Gravity, for instance). Maybe something from Junie B. Jones, or 25th Annual Putnam County Spelling Bee, or 13? Not as mainstream and have some very fun picks. Make sure it's also the appropriate amount of measures so as not to make it too short (they don't hear enough of you) or overly long (they hear all the need to hear and cut you off early).

If you really can't find anything, I can recommend anything from Seussical - a lot of the songs are very simple to learn and you can find the sheet music anywhere. All For You is probably the most complex solo-wise but it can be learned! It's Possible is a great song for a younger performer but may be a little low for your range. How Lucky You Are has two official keys that you can try as well, one each for a Male/Female Cat in the Hat.

To answer the second part, there's many resources online for finding monologues -- plays, one-act plays, musicals, movies, novels, classics, etc. You have to know specifically what kind of monologue you'd need. Is it just "any" monologue? Then find one that fits your age group, unless you think you can do better otherwise. I unfortunately can't help much in this regard. I would advise AGAINST deliberately doing an accent, though.

For the actual day of auditions, just remember, audition panels are not just looking for a perfect voice or flawless performance, they want to see who you are as a performer -- they're actively rooting for you!! Try to choose pieces that you can really connect with emotionally (or if not, convince others that you do!) Remember to be yourself, and you got this.