Make it fun! When working with freshly cut flowers from your garden, try different approaches, but most importantly, keep it simple and enjoyable. As with most design projects, odd numbers are very pleasing to the eye. Also, mix it up and go with a variety of flower types.
On the subject of flower height, they can be all similar in height, or some can be taller than others (just not significantly taller so that they look out of place).
As for color, think about the color wheel. You can group warm or cool colors together for a streamlined aesthetic. If you prefer more contrast, think about pairing opposites on the wheel. White can make for a nice accent to other colors in an arrangement.