How do I set up a home network using a Mac and PC?

I have a Mac and a PC. Currently, my Mac is online through Comcast. My PC is upstairs and I want to communicate with the two online.
Assuming you are using an ethernet cable to connect your Mac to the Comcast modem and you're PC has a wireless card installed, you're in luck. Mac's give you an easy way to share you're internet connection without having to buy extra equipment -
1. Go to System Preferences (Click the Apple icon in the top left of the screen to get the drop down menu).

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2. Select the Sharing icon.
3. If the padlock icon in the bottom left of the window is looks like it's locked, click it and enter your an administrator's login info.
4. Check the box next to Internet Sharing to turn it on.
5. On the right side of the window where choose Ethernet from the Share Your Connection drop down. and check Airport under To Computers Using.

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6. If you want to set up a password (which is a good idea), click the Airport Options button where add a password and change the name of the connection if you want.
You now have a wireless network (WiFi) setup in your home without having to go out and buy a router.. Go to your PC and setup wireless networking to access it.
First, get a router. If you can run a cable upstairs, use a wired router and a wired system. If you cannot, you will need to get a wireless router and a wireless adapter for the PC
Connect the system this way (I'm assuming wireless)
Once connected, configure the router. Routers come with an install CD, but I find them more annoying than helpful. All routers have a web based interface you access using your browser. I find that the easiest way to set up a router.
After you have completed router configuration, then you need to configure your wireless.

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Enable security - Use WPA encryption - WEP has been hacked
Change your SSID
Get the wireless working on the PC. You will have to load the driver for the wireless adapter you purchased.
You should now be able to access the Internet from either machine.
I'm just trying to mess around, screw around a little with my family, but they are all on Mac and I'm on a PC. Is there any way that I can remotely access commands on their computers like the voices and whatnot?
If you want to network the machines so that you can share files between them, then you will have to load Samba on the Mac and set it up. Samba is a program that makes the Mac look like a Windows file server to the PC. It is part of the Mac "toolkit" and available under a GPL license. You can get more detailed information on setup at the Mac forums.