First make sure that you have enough bandwidth (amount of data you can transmit through you connection to the Internet in one second, usually Mbit/s). Today's standard for DSL is 3-5 Mbit/s. This is enough bandwidth for up to 7 regular users. If you connect also your TV to the network to watch movies over the Internet and some of the users play games over the Internet you may need more than 5 Mbit/s.
Next is to buy some hardware. Typical setup is to install wireless router with Ethernet RJ-45 ports (4- 8) for wired connection. If your house is large, you may also need to install Access Point to extend Wi-Fi signal (wireless signal) further to cover entire house. You may also need RJ-45 UTP cables to connect some computers or printers to the router. Some of the computers may need wireless USB adapter to receive signal from wireless router.