It might be quite good to buy a few gardening magazines and have a look at the pictures in them. Generally they will show the usual angles that flowers and so on are taken it. Practice trying to repeat shots like them and when you get good at that then play around. This is a lot better with your digital camera as you don't have to pay for each photo to be developed, only the ones you really like to be printed.
Something that you might consider (if the camera is an SLR and you have money) is getting a macro lense. This means that you can focus really close. Normally if you do get a macro lense go for one about 90-105mm.
Another small hint, try and make sure that the flowers and plants you take pictures of are in as good a condition as possible, so that there's not a disease on them etc. Do a bit of gardening and taking off dead leaves if needs be, it's not cheating!