I am a huge fan of fresh flowers and enjoy having them in my house all of the time. I prefer to make my arrangements myself that way I can pick out all the flowers and the colors that I like for my arrangements. 
I store quite a few clear, glass vases and a few additional white ones in our house. I have several different sizes and shapes to suit my needs for different types of arrangements. I recommend keeping a little collection of vases around. It makes it easier when it comes time to make your flower arrangements. 
Putting the flowers together is fun and creative. I believe there are no rules, but there are a few tips I have learned that I can share with you to make it a bit easier and more appealing to the eye. 
First, I choose where I want the arrangement to sit in my house and which vase works well for that space. 
Second, I choose a color scheme and then pick out flowers in a few different shades or complementary shades of the color I wanted. I am sure to pick some large flowers and some small flowers so that I have a variety of sizes, shapes, and colors. Picking a few flowers that hang down is also a good idea if you want some to spill over the sides a bit. Also, adding some with height adds depth to your arrangement. Next, I pick out a few filler flowers, usually in the whites and greens that will compliment my other colors.
Third, it is time to put it all together. If I am using a clear glass vase, I start with a few rocks in the bottom. I usually add about 2 inches of the rocks in the vase. Sometimes I add more if I need some additional height. I start with the larger flowers and then add smaller flowers. Next, I place taller flowers to add a bit of height. I often put these in groups of two or three. Lastly, I add additional large and small flowers as needed, and I fill the vase with more water. 
I purchase all of my flowers from a local wholesaler. I highly recommend finding a wholesaler where you live. You will always get fresh flowers and a better variety of flowers at a fair price. I currently use Greenleaf Wholesale Florists, Reno
For the tall arrangement in my photos, I used the following flowers: 
For the large square arrangement, I used the following flowers: 
Hanging Amaranthus 
Oriental Lillys
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