Fish keeping is expensive as you'll come to find out.
Go to Pets at Home, or find a place online, and buy an API Freshwater Master Test kit £18.99 i believe from P@H.
If you want to cycle properly, look up "cycling" posts that people have created, it tells you what to do. You will need pureammonia, which can be bought from boots for around £2.50 i believe, its called household ammonia and is in a white bottle, i got mine from Sunderland - but this process can take from 2-4 weeks and maybe even longer.
Basically bacteria eat ammonia, which is what fish produce in their crap, so using pure ammonia from a bottle and adding a small amount each time to your new tank will establish and feed bacteria in your filter and eventually build up a colony, this can take weeks, this bacteria eat the ammonia and release nitrite as a byproduct, therefore another type of bacteria starts to establish which uses the nitrite, and this produces nitrate which just stays in your water but is removed with water changes.
Any level of Ammonia and Nitrite are very hamrful to fish, Nitrate isnt as harmful unless it reaches extreme levels.
Im a little rusty but il type what i can remember, anyone please feel free to correct me if im wrong.
Basically cycling, you add some ammonia to the tank and then use the API test kit to measure for ammonia, you want to raise the levels to 5ppm, so keep adding until you reach that level, then basically you leave the tank, test daily and after days itl eventually start to drop, let it drop to 1ppm and add more ammonia back upto 5ppm, keep repeating this procedure, eventually itl start to drop to 1ppm within 12/24 hours, nitrite will have started to form, keep testing ammonia and now nitrite using the test kit, this will keep going up and up and will peak at an unknown value, could be 6ppm or higher, you'l never know, then it will drop, keep adding ammonia to the tank until both the ammonia and nitrite levels drop to 0 within 24 hours, and then do a water change and your tank will be fully cycled and ready to accept all the fish it can hold.
Its best to keep a note of the numbers each day.
It sounds a right fanny and can take a while, and ya cant wait to add fish, but everyone on here will recommend you to do it, it saves subjecting fish to this and then dieing.
Of course, you could always ask the bloke at Harton Aquatics if he will give you some filter media to stick in your filter, this will jump start the cycle and make it go quicker, if he gives you alot you will probably not need to cycle at all.