How do I get debian to see my wifi?

Sounds like you might be missing the driver, in particular if it is a non-free driver.

Step 1: find out what wifi hardware you have. If an internal wificard, lspci will show you what internal pci devices are installed including the wifi card. The list is long. However this line appeared near the end.

Shows that we have an Intel wifi card

If using a usb device, lsusb will identify your wifi

This shows I have a Ralink RT5370 deivce. If you have trouble idetifying it, search the internet for the ID, 148f:5370 in my example.

Step 2: Modify /etc/apt/sources.list to handle potential non-free drivers. Login a root, su, and use your prefered editor to change this line in /etc/apt/sources.list from (substitute wheezy for jessie as for your installation)
deb http://ftp.us.debian.org/debian/ jessie main
to
deb http://ftp.us.debian.org/debian/ jessie main contrib non-free
As root
# apt-get update
will load the new repositories.

Step3: Since the above lsb command identified my usb wifi device as ralink, the following command identifies the firmware driver:

Therefore as root install the identified firmware:

PCI exanple: in the case of the intel PCI internal wifi card:

Similarly intall the identified intel firmware:

Step4: I cannot remember if it was necessary to uplug/replug the usb device to load the drivers. What I do remember is that under XFCE in jessie, the above usb related steps activated the network-manager, upper right of XFCE. Prior to that, network-manager indicated the diver was missing.

For an alternate approach, read "Loading Missing Firmware" https://www.debian.org/releases/stable/i386/ch06s04.html.en . This reference describes how to load the non-free firmware at installation time. This only works in Wheezy, maybe earlier, broken in jessie as of this date. However, this approach works so well that I was able to do a network install of wheezy via the wifi. I booted on a small usb flash image and installed wheezy over the wifi network link. Moreover, the installation retained the non-free firmware, not requiring the above additional steps to get wifi.