I would get in touch with your local \link{http://www.businesslink.gov.uk/bdotg/action/home?target=http%3A//www.businesslink.gov.uk/%3Ftc%3D000KW021413522&tc=000KW021413522&ref=http%3A//www.google.co.uk/search%3Fhl%3Den%26q%3Dsmall%2Cbusiness%2Cadvisor%26btnG%3DGoogle%2CSearch%26meta%3D&domain=www.businesslink.gov.uk\Business LInk} office. You will be able to chat about legal requirements and lots of other strat up advice also. I attended a 4 day course over 4 weeks, covering the basics of tax, vat, registration sole or Ltd, accounting , business plans etc. They will probably have forms in the place to but lots can be done on line.
The basics are register with tax office as a sole trader, than keep accurate accounts for the year and fill out a self assesment form.
register to pay self employed National insurance,
You won't have to register for vat unless your turnover is over £61,000.
You might want to consider insurance at some point but if you are working from home with a small amount of stock it won't be a major concern, but it could affect your house insurance if you tried to claim for stolen equipment etc.
Have a look at the site it has guides to all this, click on the "what applys to me tab and it will ask you questions about your business and then tell you what you have to do.
Where or how are you going to be selling your cards, the market seems to be flooded with cards just now, make sure it is worth your time to make them. If it is not much more than a hobby at present I perhaps wouldn't be in to much of a hurry to register etc depends on turnover I suppose.