Converting from freewheel to cassette hubs?

1) Freewheel hubs and cassette type hubs are NOT interchangeable.  You need a completely new rear wheel - one specifically designed for a 'freehub' and cassette.  The differences are illustrated on this link...  https://www.sheldonbrown.com/free-k7.html

2) I'll assume you currently have a 7 speed.  Stick with that.  If you want to switch to an 8, 9 or 10 speed, you'll also need new derailleurs, new chain, new shifters - new everything.  Not very "cost effective".  If you're gonna drop that much $$$ - may as well buy a new bike.   

3) I say this time & time again on Yahoo Answers.  The best place to start is your LBS (local bicycle shop).  They can make recommendations & you're under no obligation to buy.  FREE ADVICE.  Imagine that.  Easy as using Google to find a bike shop.  Just type in "bicycle shop" + your zip code or postal code in the U.K.  Example: bicycle shop 63304  https://www.google.com/search?q=bicycle+shop+63304...

4) Do NOT buy anything via the internet without first checking with a certified bike mechanic.  Me personally?  I'd just pay the few extra bucks & go through a bike shop.  That way you're assured everything is right - with a guarantee and/or warranty.     

It'd really help to know specifically why you want to change to from a freewheel to a cassette. Up date your question with this information then better answers can be given along with several options.