You can start with an extension tube on whatever lens you now have. Kenko makes a reasonable, reasonably inexpensive set for the Canon mount. With a 200mm lens, you will have a great working distance, the distance between the lens and the subject.
You will, as others have said, need a tripod and, preferably, a cable release.
You can also consider adding a close-up filter, the Canon 500D. You can use the filter along with the extension tube, at some loss of sharpness.
If you have a tele-extender, you can use it with either of the above.
Finally, you can add the true macro lens. You can use the extension tubes and closeup filter with the macro lens.
At some point, you will want a flash. Outdoor macro shots of flowers are difficult in any amount of breeze, the flash can freeze the motion.
Alan