Working With Data Files and Layers
A layer is a collection of similar data, or a "slice" of a map. Roads and streams are 2 examples of layers. In your map document, you can customize the way layers appear w/o modifying the original data. You can then save these properties as a layer file so that when you open the layer file it will display in the way you customized it. This is particularly useful for class assignments where everyone in the class is working from the same dataset. Once you have a map open, adding, removing and changing the view of data files and layers will be very important.
Adding Layers
Adding layers to a map is fairly simple. To add a layer click the 
in the middle of the menu bar next to the scale OR File, Add Data. This will open a window that lets you browse to the area where your file is located. Once layers are added they show up on the map, usually on top of any previous layers.
Changing the Order of Layers
To change the order of layers click on a layer and drag it above or below other layers of interest. Layers that are higher in the view will show up on top of other layers. Think of this as a set of transparencies on an overhead projector. Any area that one layer covers will make it so you can't see data from layers that are below it. You can also turn layers on and off by clicking on the check box to the left of the layer name in the table of contents. Layers and data sets can also be grouped in the table of contents by selecting multiple ones, by right clicking then selecting group. These groups can then be reduced or expanded by clicking on the +/- to the left of the check box.
Removing Layers
To remove a layer right-click on the layer in the pane on the left. Then click "Remove". If you have previously saved a map then don't worry if you do this by accident because you can open the older version and the layer will still be there.