If you are certain that your child will be raised as a Christian and are happy to make this decision on their behalf, a Christening is the best way to celebrate their birth. If you are already a member of a local church, make an appointment to speak to the priest and get a date in the diary for the all important event. Naturally it will be held on a Sunday and may or may not form part of the usual service. The vicar may also need to know about the number of visitors, your chosen godparents and if you have any special requests or words you'd like incorporated into the ceremony.
If you've recently moved or simply want to welcome your child into the christian community (despite not being regular church-goers), you may be expected to attend church for a few weeks before your child is allowed to be christened there. Some churches are quite relaxed about attendance but in many cases, the vicar will want to see that you are committed to the faith both before and after the christening and aren't simply using it as a route to get into a good local Christian school further down the line!