Hi Sebypoly,

I'll try to help, but you'll have to read through this long post.

First off, try to re-flash the eMMC and the firmware of your N900. The steps are easy and are described here.

Note that after you re-flash the eMMC (the whole memory module, so to speak), you should reflash the device with the latest firmware. So, you'll get the update, and this will also reset the device, so you should make a backup of your important data to your computer.

After this, start the N900, and click on the status bar. (The little area at the top left which shows the time, battery, etc.) Then, click on "Internet Connection". After a few seconds, it should list all available Wi-Fi networks, and also the T-Mobile access point.

If the T-Mobile one doesn't work, you should contact T-Mobile. (Note that it may not be enabled by them by default.)
If no Wi-Fi shows up, you should try to set it up manually from Settings/Internet connections.

If you tried all the steps above and it still doesn't work, you have no other choice but to return the device to a Nokia center and ask them to replace or repair your faulty N900.