A Squarecan, welcome to the forum.
The basics of painting a plant.
Find some good reference pictures.
Observe the shape and "feel" of the plant.
Use a hard brush and paint out the basic shape of the stem or trunk.
Lock the transpancy of the layer and shade the stem or trunk.
Now add the highlights.
Move on to the leaves and do the same thing.
Shade the stem again if leaves cast shadows.
Then do the same procedure for the flowers.
Sorry to be vague but the specific techniques for painting a plant will vary according to the plant you are trying to paint. Using a photo on your base layer for reference will be very helpful the first couple times you paint until it "clicks" for you. At which point you'll no longer need the photo reference.