go to your local tax office and talk to them, they are surprisingly friendly and not out to get u, (unless you are a self-employed joiner and then apparently they are definately out to ruin your life)
If you're an employee, use the link posted by WoWo (or ask me to do some more sums!).
If you're self-employed, post again (stating the nature of your business) and I'll try to suggest some things you should be claiming for.