Depending on your budget and type of bike it'll slso be worth looking through gumtree and eBay. There's plenty of people selling refurbished bikes in and around Cardiff at decent prices and if you decide it's not for you you won't lose too much money selling it on.
It can be quite daunting taking to the roads so try the taff trail - it's lovely and stress free.
One last thing - buy some decent shorts.