Garden Photography is one of the easiest topics to find subjects for. Just look out your back window., you should always find something in bloom.
Look for leading lines, a path up to a shed, an edge of border, anything that will lead viewers eye to the end destination in the picture.
For close-up images of flowers make sure that you are steady, focus as close as you can to the flower and shoot.
Don't try and shoot when it's windy unless you want a real challenge on your hand. Trust me I've tried it too many times and it's not worth the hassle.
For lighting try and use a bit of flat tin foil to bounce light onto the plant with out using flash.
Try a range of different angles and find what you like.
Examples are shoot the flower from behind rather than from the front.
And the most important thing is to keep shooting!