The altar tile or paten is a disk placed in the center of the altar where offerings and other tools, including the water and salt dish are placed to consecrate them.
It can be of wood, metal, or glass. What ever symbol you choose should be one that feels right to you. Myself, personally, I have one made from copper and silver inlay that is in the shape of a simple pentagram but in my coven we have several different ones we use during group rituals. A wooden one, a silver one, and even the type of pentacle on them is different and is used for various different sabbats and rituals. This should be your first tool consecrated as it is the focal point for centering your energies and will be the central tool for consecrating the rest of your tools.
Different traditions have their own requirements to consider. I am an eclectic witch so I have no demanding type for my students and suggest if they wish to have a particular type then that should be what they go with. If you wish to follow a particular tradition and want to know about theirs I suggest seeking out a coven of that tradition and seeking initiation with them. I am not an expert on one particular tradition. My coven is a group of eclectic witches who gather to create a collective knowledge and to learn and draw information from each other.
Offerings should be placed on top of the altar tile while it is blessed and offered up. As the challis full of wine is being offered and blessed it should also be sat on top of it. So when selecting your altar tile bare this in mind. Select one for which you can sit things level on top of. The altar tile is an earth symbol.