"I want to perform! But what..."

Monologues and Scenes:

If you want the ideal solution to finding great monologues and scenes, it's to constantly read plays. Make note of a character you want to play as soon as you find one, and compile a file of these notes. When the time comes to choose material, you'll have a stack of suggestions waiting for you.
If you don't have time to read plays constantly, or nothing stuck out to you from the ones you read recently, it's time to go to the books. There are monologue and scene books at most public libraries, but here's the key to that: Choose a book that has material from published plays. Don't bother with one of those "for kids, by kids" collections. They'll only get disqualified.
Search the internet. One website to check is www.actorpoint.com. Just be careful, again, that these sites are giving you published stuff. Make sure there's a play to match the scene/monologue.
Ask people! Don't be scared to contact former thespians for material they've done and liked. After all, 90% of being original is knowing who to copy. Hint: Start making notes of stuff you like at this year's district and state so you can do it next year!

The Musical Categories:

To find anthologies of musical songs, your best bet will be to go to an actual music store. Here, you will find books with songs grouped according to gender and voice type. Some books will even go so far as to have belting songs and head songs separated. The really good ones will also include an accompaniment CD so you don't have to find someone to play your music. They're a bit pricey (starting at about $16.00), but they can last you a while if you choose one with lots of interesting material.
Most of the guidelines above also apply to songs, except for perhaps reading musicals all the time. I'm not sure that's practical. But go see them every chance you get!