1. What you are trying to accomplish.
2. What supplies you have access to.
Spells are only as simple or complex as you make them. After all, it is a way of focusing your mind and energy on your intent and directing it. Whether you are casting to draw love into your life, to protect your home, to bind someone etc., that desire should be the focal point of your spell.
You must be careful with your wording and focus. If your goal is to draw love, do not mention lonliness or think about how you have been alone By doing so you would just attract more of what you do not want, lonliness.
You can incorporate as many or few items as you want into your spell. Just make sure you know the properties of what your working with. You would not want to use conflicting items.
You should also also take into consideration what "type" of magic your working with. For example, if your using sympathetic magic you would use personal effects, an effigy of the person, or something that represents what your trying to accomplish If your working within a certain pantheon, you may wish to use items that represent your God(ess). If working mainly with the elements, using items that represent them or the elements themselves would be beneficial.
After deciding your goal and any items you want to use, what are you going to do with them? Do you wish to shout your intent to the universe and cast some herbs into a fire to let them float off on the wind? Do you want to say a quiet prayer and meditate to the light of a candle or an image? The choice is yours to make. As long as you keep your focus and visualize the results of your spell as if they already came to fruition, your likely to suceede.