how do i join a wireless network with my airport express

Use Network preferences in System Preferences.

Use the Airport Utility.

Do you want to connect your Mac or iOS device to an Airport Express?

Kappy has that advice to use System Preferences -> Network

Or do you want to get an Airport Express to connect to a WiFi network so it can be a WiFi to Ethernet Bridge?

deggie is correct that you configure the Airport Express via Applications -> Utilities -> Airport Utility, or if using an iOS device, the Airport Utility app you download from the iTunes App Store.

Or do you want to setup the Airport Express as a WiFi extender?

That is a bit more difficult as it depends on what is the primary WiFi device. If that is another Apple Airport Extreme or Airport Express or Time Capsule, then it is easy and again you use the Airport Utility. If it is some other vendors WiFi base station, chances are is may not work, but you can try via the Airport Utility.

Or "Other"?

Once we know exactly what, it will be easier to suggest a solution.

Thanks, but we still do not know what you want the AirPort Express to do. What you want it to do will determine how it is set up. What service will the AirPort Express perform on your network?

If you want the AirPort Express to connect to your modem/router using wireless......then you must set it up that way, using wireless. Do NOT use an Ethernet cable for the setup.

However, the Express cannot wirelessly "extend" the signal from the Visionet modem/router if that is what you are asking. The "extend" feature can only be used with another Apple wireless router.

It is the problem.....IF.....you are trying to get the Express to wirelessly extend the signal from your modem/router. It can only do this if the wireless signal is provided by another Apple router.

If you leave the Ethernet cable connected at all times, you can set up the AirPort Express to provide more wireless coverage. If that is what you want to do, and need some tips on how to set up the AirPort Express to do that, we can help.

As Bob Timmons says, you can use ethernet to extend your network creating a Roaming network, by configuring the Airport Express as a "Bridge" and then giving it the same SSID (WiFi name) and the same WiFi Password.

If you cannot run ethernet to the part of your house where you need the network extended, you could use PowerLine Ethernet Adaptors which will use your home wiring to give you ethernet access in another part of your home.