for keeping my books I just us Spreadsheets
As for your question you asked what needs to be kept for sales and expenses?
In a short answer everything, everything you make should be documented, if you only take paypal then printing off a months statement is enough.
As for expenses you keep every receipt and every invoice that relates to your business.
In the UK its just a matter of sending off your income, expenses and profit/loss as totals, I assume its the same in other countries? But you should always have evidence of how you got to those figures in case they do a more in depth check. My mum has been running her own businesses for about 20 years and has never been asked to supply proof of her income/expenses, but maybe she's just lucky.
In the UK I believe you can use broadband fees for expenses (after all who can run an online company without access to the internet) but as mentioned before you should pick up a book.
I am lucky enough that my mum was a book keeper (many years ago but the same rules still apply) I done a short course on book keeping, so we know enough to keep ourselves plodding along (but I wouldn't be confortable giving advice to someone else, especially one in another country)We also have an accountant who is a friend of the family, so he doesn't mind answering questions and giving us advice for free.
There are various websites that claim to have accountants answering questions, you could maybe look into them.
If you want a face to face explanation of what you can and can't use, I would advice you to make an appointment with an accountant, he could advice you what you can and can't use, and then you can keep the books yourself, although he will probably charge you for the appointment.