On freelanceadvisor.co.uk: The definitive freelancer accountancy glossary and What is a sole trader?.
The HMRC website offers lots of assistance. Join in a couple of HMRC self-employed webinars such as "Self-employment – what you need to know" and "Cash Basis". They also have an online course for starting your own business.
You will also find more help on the GOV.UK website about working for yourself: Set up as a sole trader - GOV.UK.
Invoicing legalities at HMRC: Invoicing and taking payment from customers: the law. They also have an Introduction to Self Assessment and Registering for Self Assessment, and an introduction to Record keeping (individuals and directors).
Make sure you know how much tax and National Insurance you need to pay for 2013/14, and what the standard tax allowance is: Freelancer tax rates for the new financial year. There is also a Self-employed ready reckoner where you can enter your monthly (or weekly) net profit, and be told how much tax and N.I. you will have to set aside.
To maintain your entitlement to a state pension, it's important that you keep paying Class 2 National Insurance (about £140 in 2013-14). Here's the HMRC guide on how to pay self-employed Class 2 National Insurance. They also let you check your 18-character NIC reference number. You can set up a direct debit instruction, or pay direct to the correct HMRC bank account. The phone number for self-employed NI questions is 0300 200 3505.
The HMRC also has a useful Giving your business the best start with tax PDF guide. You should also check out their HM Revenue & Customs channel on Youtube, such as this important one about effective record-keeping:
Freelance Rose photo by Georges Seguin (Okki) (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons