Don't overclock it for at least a few months. You want to be able to return it if it's defective in some way, and you can't if you overclock it because the warranty is voided. Read the manual and learn how to do the stuff you can do. You don't need overclocking for multitasking, it's just something you will be able to do.

Buying an extra battery isn't a bad idea, and buying a desktop charger is also handy. But there is probably no big rush to do either.