Start using a good editing program: Photoshop, Paintshop Pro, etc...
Here's an extremely quick touch up I did to one of your photos (I hope you don't mind) :
Quick touch up:
Far from a good edit (in my self- critiquing view I should have done that center area of the plants differently, and probably not as vibrant ) , but it was done very fast and it's just to give you an idea of what can be done/fixed.
Also, if your using the kit lens, you might want to upgrade. Nikon has a nicely priced prime lens for under $200: Nikon 35mm f/1.8G AF-S DX Lens