The pdf document below is an excellent Introduction to Racing by Matt Winston, published in the Go-Ride magazine.

This article is aimed at Youth riders, and has information for the rider and their helpers and parents - what to expect at your first race, how to prepare for it, hints and tips, rider categories explained and more.

In the document Matt suggests a 46 chain ring with various smallest sprocket sizes for each Youth category. It very much depends on your bike and whether you want to spend money on specific chainrings and rear cassettes, or make do with your bike as it is. More serious racers will use specific combinations. A 46 chainring may require a compact chainset. Smaller chainrings can be used with a wider range 10 speed cassette (locked out as required).

An Introduction to Racing