Hello & Welcome
first what is important is to 'cycle' your tank, meaning to start it so that beneficial, needed bacteria can grow and develop that are necessary for your fish (and their survival) detailed explnation here http://www.tropicalfishkeeping.com/f...ium-cycle-252/
Next step is to determine what kind of water you have, which should incl the pH and hardness. For this any pet store of your choice has liquid test kit's for sale (here commonly by the brand API, but I donno if the UK has different brand names there). The clear advantage of these set's is helps you to measure your water accurate (vs the strips they sell are very inaccrate results) and also offer test's within the kid such as Nitrate & Nitrie and Ammonia (these 3 are the need-to know one's while cycling a tank and that will also show you when its done cycling and/ or when levels get too high and could harm your fish).
So in the long run and for starting your tank, this is a VERY good investment.
Then comes the consideration if you'd like live plants in there, if so select the plants you like and plant them in your tank when you first fill it with water. Personally I pref live one's over plastic simplye because it gives you a nice & natural look.
Last but not least: Fish
Now there's fish that prefer higher pH and harder water, while other (such as Tetra) like lower pH and softer water, to make this decision on what you like and what will thrive healthy in your tank, test the water from your tap. They can handle a slight up or down from ther natural habitat but not a LARGE swing.
If you already have seen fish at your store that you like and recall names a very good page to read up on their needs is here (use search function on left side to enter names) Tools - Seriously Fish
To select different fish what you'd want to watch for is:
- What water do I have and what do the fish ideally need?
- What tanks size do I have and what do fish need? (Don't get a fish that will outgrow your 60l tank)
- The fish I want is it aggrassive and do I want to house it with let's say a calm fish (such as Tetra)
Hope this helps for the moment, please post and and all questions any time, there's a lot of very knowledgable & helpful people on here that will gladly help you hrought the whole process (which really seems more complicated right now maybe then it really is)