Unless you are working within a specific Tradition that has a certain way of setting up your altar there are no rules about where it should be placed or what you should put upon it. The altar is the place where you interact with the Gods so it should represent that interaction to you in whatever way you find best.
"Traditionally" an altar would hold a representation of the God and Goddess, symbols of the four Elements, some candles to see by, and your working tools.
In many Traditions the altar is placed either in the North or sometimes the East of a room. But again this is not a hard and fast rule. If the way your room is set only gives you room to put your altar in the West then do it that way. We Witches may be traditional, but we are also practical and we can work with what we are given.
Your altar cloth would typically be made of a natural fabric such as cotton. When starting out check out places such as Walmart for inexpensive table cloths. I recommend inexpensive because sooner or later it's going to get dirty or have candle wax spilled on it and you're going to want to replace it.
For representations of the God and Goddess you can either use candles or you can use statues of the Deities. In the beginning it is probably easier and less expensive to go with a couple of pillar candles. The Goddess candle goes on the left upper corner of the altar and the God candle on the upper right.
To represent the Elements you can use the following:
East/Air - incense
South/Fire - red candle
West/Water - chalice or cup of water
North/Earth - a small dish of salt or dirt
There's a pretty good you-tube video at http://www.youtube.com/watch?v=D1a9mAaESiQ that might give you some ideas as well.
Now, since you don't want the altar to be particularly noticeable you might start out with a small space on which you place some candles and some plants. Then when you are actually working at the altar you could add other items pertinent to that working and then remove them when you are done.
One last note, having an altar will NOT make your spells more powerful. The altar isn't about working spells, it's about having a space where you interact with the Gods to honor them. So if you're creating an altar strictly for spell work because you believe it makes your spells more powerful you could actually skip the altar altogether if it would cause you problems.