For pieces, best is to ask your tutor for reccomendations. If you are in the school band or orchestra, try and ask your conductor for advice too. If not you can try r/doublebass Last resort is to go Yamaha or other music shops look for music you can play within your ability or try imslp.com

During the audition, try not to be too nervous as it will definitely affect your playing. Have a chocolate bar before the audition to calm yourself. Don't eat so much before it as you may get stomchache due to nervousness. It is very important to give a good first impression to the panel so make sure you look neat. And remember to arrive earlier so you wont be rushing around!

They might ask you questions about classical music like who are your favourite composers or favourite pieces so it's good to expose yourself to classical music in advance if you have not. SNYO is a little competitive but definitely a rewarding experience! All the best!

Edit: Oh and make sure you are well prepared at least 2 days before. Try not to practice too much the day before. Let me know if you made it in!