WN, welcome to WW, and congrats on selling your article!
You only have to charge VAT on things if your
turnover from self-employment (i.e. your being-a-writer) is over a certain threshold. I think it's about £65,000 a year. So it sounds as if you don't need to worry. It's completely separate from income tax, which you pay on the profits of your business: i.e. the money you make after you've deduct all the expenses of the business (paper, postage, ink, research costs etc. etc.).
The Society of Authors has good basic leaflets on tax and VAT for writers: they're free if you're a member (you can join if you have a publishing contract, but I'm not sure what the arrangement if is the contracts are for articles), or a few pounds if you're not.
http://www.societyofauthors.org/index.html
(Back to the VAT, just in case you sell tons in the near future. If you do find you're selling things, there's quite a wide band of turnover under the compulsory limit when you can register if you want to. It is actually advantageous, if most of the people you deal with are VAT registered. They don't mind you adding 15%, because they claim it back. And you don't have to account for each little 15%, you just pay a percentage of your turnover, quarterly (everything to do with VAT is quarterly, including letting them know if you're approaching the threshold), to the VAT-man. That percentage for writers is I think 11% (it depends on the industry), so you make 4% extra on everything, if you don't mind doing the VAT paperwork.)
But it sounds as if you don't have to worry about that at all, so don't panic.
Emma
<Added>Meant to say that accounting packages for the computer will mostly cope with VAT. I've just bought QuickBooks, and it's dead easy to use. Might even brace myself and register.
And b) I'm wrong about VAT accounting being quarterly - if you're a small business, which writers almost always are, you can do it annually.