whee If that table is already for you, and you've got your own figurines on it then it techincally is already your altar. It doesn't have to be made a 'set way' - don't get confused by that. If it harms none, then anything is fine: although generally little tidbits to represent the elements and the lord and lady help. If you wish to have them in their coresponding directions then the most basic would be:
north - earth - flowers, salt, soil, green candle, green ribbon, green pencil.
east - air - feathers, incense, wand (probably not a good idea for you, but twig type looking things can be found in any craft store these days - and baskets full of flowers and fruit. I wonder where that idea came from..) Anyway. Yellow candle, yellow pencil, and picture of something with wings..
south - fire - candles, red, a picture of fire, matches
west - water - sea shells (Jess, you stole my censor idea
whee ), blue, water, blue water, picture of water, salt..
Those aren't hard and fast rules. Be creative. It doesn't have to look anything like the element to anything else - just something that will represent the element presonally for yourself.
To find out the crystals correspondences, the link can be found
hereGood luck, and have fun!