If you go to the garden centre, there will be penty of plants in flower. Those can go in straight away. Plants in pots can go in more-or-less when you like, though if you leave it much later than this, they won't have much time to settle in and make root before everything starts to die back. You need to think about whether your soil is dry and light or wet or clayey and get advice (or look in a gardening book) for plants that suit those conditions, as well as how much sun the border gets and how tall you want the plants to be. Do you want shrubs or flowering perennials or annuals? The first two are more permanent but the third, annuals will give lots of brilliant colour. There's one thing, though - if you put something in that gets too tall or doesn't like the condtions, you can always move it and gather gardening experience. Have fun!
