There are two things you need to bear in mind when setting your stops:
a) before you enter the trade, work out where you will exit IF the trade goes against you. Always protect your risk and the profits will take care of themselves. Work out how much this will cost you in real money. When you are more advanced you can work this out as a percentage. Decide if you are really willing to take this loss; if not, don't enter the trade and wait for the next one.
b) position it so that when the price comes down to take out all those traders with stops, yours does not get hit along with all the others. So leave sufficient space below the 00s (£1.00, £18.00, £19.00 etc) and previous troughs in the price. Unfortunately there is no easy formula; you just need to get a feeling for where the most appropriate place is.