I have been cutting flowers for small arrangements all summer, allowing my creativity free run to combine whatever is blooming by the color theme/mood I want to explore. I think it helps to know basic floral design: line flowers create a framework, accent blooms move the eye and fillers/foliage then carry out the rest. But not necessarily. Remember the cardinal rule, relax and HAVE FUN! :)

Modern arrangements usually have only one or two types of flowers, modern flower arranging is mostly artifice.

The really cool thing is cutting bunches of flowers and then combining them in a vase, because they would never grow side by side, for example, shave loving hosta leaves to cover the cut stems of sun loving agastache and conflowers. Maybe throw in a daylily, only lasts one day. Some calamint or lady's mantle for filler.

I took a class at the local community college, it was so fun.

Steve
I picked up a copy of Florist Reviews' "Flower Arranging" which is a very handy resource to have around.
I do okay on my own for arranging but I wanted a little bit more direction so just took a two week full time course at the Benz School of Floral Design at Texas A&M Uni. Maybe a similar course would be of use to you. There are loads of books out there, hope others will chime in on their favourites too.
Cheers Kim - BFFF