You should be able to do something with a reversing ring with the lenses you have. This example is based on a 70-300mm and 50mm so should apply pretty well to your gear.
http://www.dpchallenge.com/tutorial.php?TUTORIAL_ID=40
Failing that close up filters are a cheap, easy way to get started on Macro. Pretty effective too.
And to prove that this series of pics was taken just using a cheap set of close up filters off Ebay for about £15.00 (+1, +2, +4. +10 dioptres) fitted to a kit lens on a Canon 350D
1. A Ladybird Larva taken using a +4 dioptre filter
2. Insect and Flower using a +4 dioptre filter.
3. Fly on Nettle Flower using a +10 dioptre filter.
Smaller filters work much better than large filters so the 18-55mm kit lens on th Canon 350D worked very well.
As usual with all macro work the DOF is always very small so be prepared to use my technique: take hundreds of shots and throw away a lot of duff ones !
And turn off AF - it is useless on macro shots - use the preset focus and rocking back and forth technique.
The photos shown here were taken without flash but for most shots flash is preferable as long as you can balance it with the daylight which normally means working at about 1/200 sec for the flash and then setting the aperture as needed (usually a very small one to help with the DOF).
And as has already been said - practice, practice, practice - in fact a good idea is to do what I often do and bring home a lot of little flowers etc and photograph them indoors using a tripod to get a feel for the subject.
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