How do I get internet to a computer with no wireless capability?

Basically you will connect an Ethernet cable from the LAN (opposing arrows) port on the AirPort Express to the Ethernet port on the iMac. You will then set up the Express to join your existing wireless network. In this configuration, the Express' ports will be active and the iMac would get Internet access.

So the key here is to configure the Express properly. The following Apple Support article will give you the basics.

Alternatively, you can always return the AirPort Express, and try a USB wifi adapter.

For instance:

http://www.amazon.com/TP-LINK-TL-WN725N-Wireless-Adapter-150Mbps/dp/B008IFXQFU/r ef=sr_1_1?s=pc&ie=UTF8&qid=1457476515&sr…

Should be easier to setup for what you want.

I'm not sure what to do now

We can provide the steps that you need to perform to try to configure the AirPort Express.

We are assuming that you have a wireless router there, which is providing a wireless signal to your home. Is that correct? Is this by chance another Apple router?

Will you be using the iPad that you list in your profile to set up the AirPort Express......or another device?

You do have 90 days of free phone support from Apple as well. it's your call, but things might be easier having another person on the phone to guide you through. An online forum is not quite the same as a phone call.

The Arris DG1670A is a combination modem & wireless router or gateway device. The support article instructions, that I provided in my earlier reply, should work to get the new Express connected to the Arris' wireless network as a client. This is what you will want so that the Express will provide an active Ethernet port for the iMac.

Note: You will not need to make any configuration changes on the Arris gateway.

I was asking if you wanted to use the iPad that you listed in your profile to set up the AirPort Express. Did not know that you had a MacBook Pro, since that info did not appear in your profile. We can use the MacBook Pro if you want.

One of the different and sometimes difficult things about using an online support forum is understanding that the folks here who are trying to help only know as much about your network as you can tell them. We can't see your network, or know what you might be thinking.

So, we may appear to ask dumb questions when the answer is obvious to you.....but totally unknown to us. Some patience and trial and error is likely going to be required to make progress. Thanks for understanding.

So, if you want to proceed........

We understand that you want to use the MacBook Pro to set up the AirPort Express, correct?

Have you tried to configure the AirPort Express at all, or make any changes to the AirPort Express?