A duplicate invoice book / receipt book is handy to have for your customers, perhaps with a rubber stamp with your business name etc.
All income and outgoings can and will need to be recorded on a simple spread sheet, as you will need these figures for your tax return. If you drop me an email ( address in profile) I will send you some I have used.
Don't forget to register as self employed with HMRC within 3 months of starting your business.
Agree with Neil, try and build up in the private/domestic side first.
Good luck with your business
Steve