[collapsed]

I recently changed my ISP ; my old one gave me a modem with 4 ethernet ports and wireless capabilities. My new one uses a different modem with only one port, which will be connected to a router as I have 2 desktop PCs. The router I'm currently using has no wireless capabilities so it won't be of much help.
I also want wireless internet to use with my playbook...How can I use my old modem as a router so that I can get wifi on my tablet/phone?

[hubermania]

You don't want to use your old modem as a router, you want to use it as a switch. (Here's the difference.) The new modem will function as the sole router on your LAN. FYI it would be helpful if you post the make/model of your old and new modem devices. Absent model information, the process is generally as follows:

Connect your PCs to the old modem LAN ports. Login on your old modem and switch off DHCP (the new modem will function as DHCP provider). If your old modem has a setting to change the mode from router to switch, do so. Now connect one of the remaining LAN ports to your new modem. Hopefully at this point your old modem now functions as a switch instead of a router, and you should be able to access the web using both Ethernet and Wi-Fi.