An easy way to fill your pots is to use the Thriller, filler, & spiller rule.
The thriller is the tallest and most vibrant plant. It is the eye catcher of the pot and should have the most vibrant color and blooms.
The filler should fill up the extra space in the pot and should be shorter than the thriller.
The spiller should a plant that cascades or trails to drape over the side of the pot.