Before you embark on a keirin or bike building adventure, you can do some exercise on the web.
For those who cannot afford a real fixed bike: Pedalmafia makes miniature toy bikes. Their website actually offers enough play in itself. You can puzzle with all possible bike parts and compose the bike of your dreams. So doing, you will quickly discover whether you know how to build a bike at all.
Keirin for dummies is to be found here. Entertainment guaranteed, but the learning will depend on your level of Japanese.