The government seems to presume that everyone knows where the HMRC homepage is - but here it is anyway. Theoretically you can find everything you need to know from here, but if not you can ask your friendly local neighbourhood accountant.
Xero book-keeping software has become popular with accountants and clients (a tricky combination). Like other similar software it works from a browser and links direct to online banking rather than having to key eveything in manually. It costs a monthly subscription which either the client or the accountant can pay, and links to many other apps.