I agree -- tenohfive has given you some very good specific recommendations.
I'll just add some more general stuff for your extra reading. I'm in the US so I use gallons, but there is an aquarium calculator up there at the top of the web forum page if you need it. Tenohfive's recommendation there is about a 16 gallon and that falls right in a good place among sizes that are generally good for beginners. Usually 10g, 15g, 20g and even 28 or 29 gallon are recommended, the 15 and 20 ones being particularly nicely sized.
As the water volume gets smaller, down below 10 gallons, the changes that can happen to the tank from water chemistry, temperature changes and just quick changes in general can end up being a bit more difficult for beginners. This can be problematic and confusing to beginners who sometimes think a smaller tank will automatically be easier.
As water volume gets larger, the water chemistry and stability just keeps getting more stable and easier to deal with. Changes are slow to large volumes of water, giving the new aquarists increased time to see that a change is happening and increased time to react and do something. On the other hand, as a tank gets bigger, say 30g and beyond, it definately becomes a much bigger thing to deal with. The water changes become a bigger bit of work, reaching down in the tank can become harder and of course the expense of equipment can grow by a great amount. This hobby is one which can be more expensive that beginners often realize when they get into it.
Often its good to think about what locations you have available for keeping a tank. They should be situated out of sunlight and not be in high traffic areas where they might get bumped or hit. And of course the floor needs to be able to handle the unusual weight. So these things may have an impact of the decision about tank size.
Good luck on your size deliberations. Another good thing to learn about early are aquarium filters and the media that go in them. This forum can help you learn some better things about that than you might get, just looking at them in a store. Then when you are past the filter decisions, an important thing to get into is the topic of the Nitrogen Cycle and fishless cycling as was already mentioned.
Its an extreme advantage to have guidence from a forum like this and learn about fishless cycling prior to setting up a tank as a beginner.
~~waterdrop~~
i mean what size tank btw also i live in the uk not sure if thats got anything do with the sizes but arent the us sizes diffrent to uk sizes in tanks??? hmmm not 2 sure? 
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