I assume you want to get a train to Central London, rather than London City Airport? At the time of booking an easyJet flight on the web, you can click on an icon that will allow you to buy discounted Thameslink tickets. This is only available at the time of booking your flight, otherwise you have to pay the normal fare. If you really want to book in advance try www.qjump.co.uk. However, if you book using qjump it'll be the same price you'd pay if you purchased the ticket at Luton Airport Station, so there isn't really much point. In London, Thameslink serves Kings Cross, Farringdon, St Pauls, Blackfriars and London Bridge. Trains run every 15 minutes - board a Brighton service. If you want Victoria, I'd recommend the Green Line 757 Coach from Luton Airport, it runs every 20 minutes.