The thing to realise about (most) flowers is that much of their astonishing colour comes from the way the petals treat light. Many flowers are translucent.
I take advantage of that translucence. First of all, I photograph them in good light, on bright sunny days. Then I (usually) place the camera so the flower is roughly between it and the sun. Depending on the type of effect you seek, you might prefer to have the sun in the shot, or outside it. It is that sunlight coming through the petals that makes those petals look brilliant.
You can shoot from behind the flower, from underneath it, etc., so long as you give the petals the chance to transform the light. This doesn’t work with all flowers, particularly those with dark colours.
The quality of sunlight changes over the course of a day, so you may find some flowers provide better results at particular times. Now, for me, this is a luxury, because the quality of light where I live means I’m less likely to be bathed in beautiful sunbeams, and more likely to get a gentle soaking.
I usually use a 85mm or 50mm lens, less often a wide–angle: so long as your lens can fill a frame with a relatively small object, you’ll be fine.