The athame is a ceremonial dagger, with a double-edged blade and usually a black or natural wood handle.
It is considered a masculine symbol. It can be used as an air symbol or a fire symbol depending on the tradition you use. Blades may be made of iron or steel, however, if you plan to use the blade in faery magic iron is not a good idea. Although it is a knife it is not used for physical cutting, [however some traditions do allow it to be used to cut magickal food or to carve symbols in candles and runes]. Its edges should be dulled so as not to draw blood if that should happen the tool is considered tainted.
Athames are also called ritual knives. They can be used to cast a circle, open and close quarters, cleanse and empower items or direct energy. You may personalize your athame by carving specific symbols in the handle to tie the magickal intention of the blade to the practitioner, the elements or if part of a group to the group. You can empower your athame by rubbing a magnet, crystal or lodestone over the blade several times to magnify its power.
An athame is a tool that is used to channel and focus energy and is used most often during the casting and the banishing of the magickal circle. Held before the caster it is used to draw a pentagram[5 pointed star] at each of the cardinal points starting at the Eastern quarter and proceeding clockwise [deosil] stopping to draw the invoking pentagram continuing around the circle until the final quarter is reached [North if you started in the East]. The Athame is used in the same manner to banish the circle, you may travel the circle counterclockwise [widdershin] drawing the banishing pentagram in the air. The Athame is also used to open the circle without breaking it.
The athame has other uses as well it can of course direct the flow of energy like a magickal laser pointer taking the gathered energy you have and directing it into something such as if you use your athame to carve your candle then you may direct the raised energy through the athame into the candle and the carving of words and symbols on it. This tool also stores energy the more it is used in your rituals the more it stores your energy.
You may find a selection of Athames here