How do I catch it?

The flu is communicated by 'respiratory secretions' - that is, from coughs, sneezes, and direct contact with any 'products' of those things (such as mucus and saliva).

You can be infected with the virus 1 to 3 days after being in contact with the disease and not show any symptoms. However, you'll be infectious for 24 hours before the first symptoms appear, and up to a week after they develop.