If it is a traditional system it will have a cold water "rising main" which normally feeds the kitchen cold tap (potable water) and a header tank in the attic, or high up in the top floor. The header tank feeds everything else and until it is fully drained (Normally about 10-15 gallons in old money) the system will still have water in it. Read the instructions carefully before fitting your new tap.
There is a reasonably good DIY guide here:
http://www.diyfixit.co.uk/plumbing/
For replacing taps under the rim of a bath or sink you may find you need a specialist tool called a shetack wrench. Quite cheap from a plumbing supplier or DIY shop.Take your time and have fun!