Getting tickets is simple but can be expensive if you don't look around. The cheapest way is to buy them straight from the theatre in person - you avoid the booking fee and can select which seats you have a lot more easily. This is, however, only more beneficial if you are able to get to the theatre easily.
Otherwise there area few internet services that can be used
Ticketmaster: Expensive booking and processing fees but is very reliable. The website can be found here: www.ticketmaster.co.uk
Express events: If you're going to the show monday - thursday some fantasic discounts can be found here: www.expressevents.com