If you are just starting up I would recommend a normal invoice book that you buy in W H Smiths and get a rubber stamp with your company details on it.
That way you can invoice the customer on completion and get paid straight away. If you start sending out invoices a pound to a penny you are going to end up chasing non payments and late payments. Do you really want that at any time never mind when you are just starting.
If you end up getting council, housing assoc, estate agents and that sort of client on a regular basis then I would suggest you invest in sage accts plus. This program allows you to enter the price you pay for products and the price you sell them at, you can send out quotes and invoices with your own logo etc on it.
Hope that helps
Chris