If you do not need to be too posh, get a duplicate book and a rubber stamp with your details on. Stamp each page and then write on the customers name, the date, what the work is and how much you have charged.
George is a good program for organising window cleaners as it is written for them. That has a invoice bit. A really good £50 worth. www. georgesystems.co.uk
If not, design a letterhead and use printoffs, they do need to be numbered or the taxman will not like it.
If you get a cheap accounting program this will probably have an invoice section to print invoices and record them, then when you have been paid it will mark it off and do your accounts. Speak to your accountant which program he uses, and you should have an accountant. ( unless youre completely in the black economy)
If you get to the dizzy heights of VAT that is a different ball game.
Trevor