The internet is a great source of tax and business related information, not all of it is up to date and should be relied on! A well worded google search on a particular topic is bound to produce a long list of potential links so I do not seek to be at all prescriptive in providing these useful links below.
HMRC (www.hmrc.gov.uk)
HMRC have been improving their website and you are able to search within the website with ease to find all you need to know about, from Air Passenger Duty to VAT. It is however worth providing two links which will enable you to make tax payments quickly and efficiently.
For self assessment, go to www.hmrc.gov.uk/payinghmrc/selfassessment.htm
For companies, go to www.hmrc.gov.uk/payinghmrc/corporationtax.htm
Companies House (www.companieshouse.gov.uk)
It is now possible to set up a new company online and Companies House make this a relatively painless process www.companieshouse.gov.uk/infoAndGuide/companyRegistration.shtml).
There are additional compliance requirements for companies and LLPs and it is possible to file many of the required forms on line (see www.companieshouse.gov.uk/forms/introduction.shtml).
Source of government guidance on setting up a business ( www.gov.uk/browse/business)
This is a good source of general information for those looking to start up a new business including the self-employed.