Please be careful when shooting in your back garden! Where is the nearest habited area behind your targets? To give you an idea,
GNAS does not allow you to shoot unless you have 150 yards clear ground from your shooting line to the back of your range!
If you're not a member of any association, you will not be insured for any accidents, and even if you ARE a member, shooting alone in your garden may invalidate any insurance. Please also check your local bye-laws.
Beyond that, I echo what the others say, find a club and someone who knows what they're doing.