you log in to the access point with a web browser (usually http://192.168.1.1 or 192.168.0.1 you can find the default password for the web interface online) and put a password on the wireless. a wep key is the most popular type of key.
You can share a USB modem internet to other computers on the network by simply creating a wireless network with the comp where the modem is connected and then connecting the other computers to the network via the wireless network that you created.
You will need a router, wireless adapter, to have a network connecting computers
Network
Many Wireless routers and switches have few inbuilt switching ports capable of connecting Wired computers to Wireless Computers on the same network to connect wired Computers also alongwith. (Not necessarily every every wireless router will be having switching ports)
You can't add wireless router to your computer. You can a wireless router to your network. If you do so, it does make your network wireless. In order to use your network wirelessly you need to have wireless adapter for all your computers you want to be connected wireless.
for connecting 4 computers you would need a "hub"(en.wikipedia.org/wiki/Network_hub), and a cat5 cable to connect each computer to the "hub". each computer would also need a "network card"(en.wikipedia.org/wiki/Network_card), alot of modern computers come with a network card integrated so you may not need to purchase it separately.
The Media Access Control - MAC Address is a number, created by the manufacturer, which identifies your computer on a network. If you have a wireless network, for example, you may want to add the MAC address for each computer you want to connect to the network, and tell the router to only allow these computers to connect, to prevent unknown computers connecting to your network.
one with no wires? wireless? which would mean you need a wireless router? i think an AD HOC network is the simplest. It allows two computers or devices to talk to each other without needing a middleman aka wireless router. as far as i know only 2 computers can be used in an AD HOC Network
There are many types of built in wireless network connections. For example, lots of phones and computers come with Bluetooth.
to split the Internet between different computers or to make it wireless if it is a wireless router
Yes you can see other computers in a wireless network, each device has it's own unique MAC-address which describes to the computer what type of device is connected to the network. Using the 'Network' feature on the computer gives the person a graphical view of who is connected to the Network, this can be done by going to: Start, 'Network'.
The easiest way to set up a new network laser printer to a network is to use wireless. The printer will simply connect to all computers on that wireless network.